Add binary builds to tahoe-lafs.org's buildbot/download page

[cypher]

I've recently put together some simple build ​scripts to help automate the creation binary distributions of Tahoe-LAFS for Windows and Mac using ​PyInstaller. Like bb-freeze, py2exe, py2app, and other related projects, with enough tinkering, Pyinstaller can be used to create self-contained distributions of python programs that can effectively be run "out of the box" by end users. Given the rather convoluted state of python packaging (and with it, the additional mess of horrors that typically comes with building Tahoe-LAFS from scratch), I think it would be desirable to have such packages available to end users as an alternative to the current offering of a manual install.

The aforementioned scripts can be found ​here (for Windows) and ​here (for OS X) and are currently being used by Gridsync's buildbot (which follows Tahoe-LAFS' upstream master branch and publishes the resultant binary packages ​here). It shouldn't be terribly difficult to port these over to tahoe-lafs.org's buildbot and I'd be happy to help out with the process (or even lend some buildslaves to it, if needed).

A few explanatory notes:

• A ".spec" file is needed to inform Pyinstaller of any required files or modules that might not otherwise be detected by analyzing the package's import statements. The one I put together for tahoe ​here pulls in the static files needed for the web UI (as well as the 'characteristic' and 'cffi' -- which, for whatever reason, aren't automatically picked up by Pyinstaller). If you use this, the specified filepaths will probably need to be changed.

• The above spec file generates one-folder bundles by choice rather than one-file binaries since the one-file approach will fail if the system tempdir is mounted with the NOEXEC flag (and one-file mode works by extracting bundled files to the system tempdir and running them from there).

• On Windows, OpenSSL will need to be installed in accordance with Daira's instructions outlined ​here. The windows script will build OpenSSL if run with the --with-openssl flag but the other dependencies specified at the top of the file will still need to be installed manually. (This is no longer necessary)

• The build scripts patch out the setuptools requirement from _auto_deps.py before running pyinstaller which allows tahoe to run frozen. Maybe this isn't necessary and maybe there's a better way to do it (I do see ​some code to this effect in _auto_deps.py) but I couldn't get tahoe running frozen without patching as init.py's check_requirement() fails with "PackagingError?: no version info for setuptools". More investigation is needed..

• Pyinstaller builds can be made reproducible (assuming the same platform, interpreter version, and architecture) by setting the PYTHONHASHSEED environment variable to a known/shared value. Note that this only affects the pythonic bits (and not, e.g., the C libraries); given the current dependencies, more work is needed to make these packages fully reproducible (but even so remains a worthy goal).

• Pyinstaller builds are forwards-compatible but not (always) backwards-compatible; it's better to build on older operating system versions if possible.