allow linking with QPL code

[zooko]

FSF long ago said that people could specially exempt their GPL-licensed project so that it could be combined with QPL-licensed code.

​http://www.gnu.org/licenses/license-list.html#QPL

I recently discovered a QPL-licensed thing that we might want to use someday. It is an SSH library built on top of Jack Lloyd's Botan library:

​http://netsieben.com/products/ssh/

(The appeal is that we could, in theory, replace Crypto++, PyCrypto?, and pyOpenSSL with Botan, NetSieben? SSH, and Botan's SSL, going from three crypto lib dependencies to one. Also Jack Lloyd is a contributor to Tahoe-LAFS so he is likely to respond to our needs quickly.)

Here is a patch that says you can combine Tahoe-LAFS with QPL-licensed code like NetSieben?. I also threw in another exception saying that you can combine Tahoe-LAFS with Eclipse Public Licence-licensed code like Eclipse. (Just in case someone wants to combine Eclipse and Tahoe-LAFS I guess...)

[zooko]

David-Sarah reviewed it on IRC, reminded me to add Apache Licence. Here it is: d826db89af6b9fc6.