Initial implementation of bencode/bdecode pod

Sorry about the length—at least half of the lines are the `xcodeproj` files, but there are still several hundred lines of implementation and several hundred lines of tests.

@vokalinteractive/ios-developers Anyone want to review some code?  (Log any time to `VOKMockUrlProtocol`.)
Yeah, in general, these methods all use the `NSError **` error-by-reference pattern to bubble errors back up to the caller.  The assertion cases, though, are to catch local logical inconsistencies.