Cleaned up Gem class, add GemCache, add vendored dep detection

- reorganized cleaned up some methods in Polisher::Gem
- Added Polisher::Gem#unpack, #each_file, and #downloaded_gem_path
- Add mechanism to cache gems so that they don't need to be retrieved
  multiple times
- Implemented Gem#vendored venderored dependency retrieval mechanism,
  work in progress
- Stub out Gem#diff method

Specs have been updated to still work though new bits need tests