Remove MIME gem dependency + Gmail::Message specs

- Remove MIME gem dependency, which is now causing gem to break since MIME gem changed it's API. Seems like this is an unused relic of this gem's past. If you run `git blame` you can see that `require 'mime/message'` was added to `lib/gmail/message.rb` in this repo's first commit, and doesn't seem to have been actually used anywhere.
- Add some initial specs for Gmail::Message class using mocks
- Rename spec helper "mock_mailbox" to "live_mailbox", since it connects to the actual Gmail service. A "mock" in the traditional sense is an object with stubbed methods.