Fixes #13: :guest_cache_dir config should get its value from the synced folders hash.

Other updates included are:
- Added default cache folder (.vagrant-butcher) to the gitignore
- Incremented the major version number to 2.0.0 since the removal of the
  guest_cache_dir is a non backwards compatible change.
- Updated the final info log to be a success message.
Merge pull request #18 from flowboard/issue-13

Fixes #13: :guest_cache_dir config should get its value from the synced folders hash.