Fix API issues

@jvlcek Please review.

This PR removes or changes the API to handle a handful of usability issues I had with the gem.  These include
- Fixing the path to the bugzilla command to work on a Mac.
- Removing the .bugzilla_credentials.yml file in favor of using accessors.  The client can choose how they want to store their creds.
- Changing the interface to be instance based instead of class based.  Callers can pass the creds and the bugzilla URI to .new.
- Changed return values of methods to not return the command for "principle of least surprise".  In order to get the last command, for debugging, I've added a last_command method.
- Changed signature of "query" to match the signature of "modify" method, allowing for a more dynamic set of options.
- Added YARD documentation to query and modify methods.