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`.)
They don't, but I think crashing with a descriptive error is the appropriate behavior during development. 

@vokal-isaac Should we return `nil` from this if the dictionary start delimiter isn't there?