Quality and extension for convert image + fixes and specs

I made a fork of reinaldomendes' fork. Reinaldomendes had added support for passing in :quality and :extension arguments.

I added specs, cleaned up the regexes, and also added the ability to set global defaults for quality and extension.

Please merge these changes in. Note that these changes are all backwards compatible.

In particular, merging the specs in will make it easier for any further development on this gem to be backwards compatible.