create internal VerifierNode/RepairerNode classes

[warner]

I'd like to have a set of VerifierNode and/or RepairerNode classes, created by the nodemaker, which implement IVerifiable and IRepairable (but not IFilesystemNode). This would be the first step towards fixing #482, by giving us an object that represents what we can do with a given verifycap/repaircap.

I'm thinking that these objects might be created with or without the integrity information. If without, it would just wrap a storage-index. It'd be nice to be able to find out what the SI represents, determine its UEB value (by consensus, not by cryptographically-ensured integrity values), and then verify/repair the file on the assumption that those shares were correct.