Quality Option Fix
- 1. _.extend expects the defaults passed in as the first argument
  - http://underscorejs.org/#extend

- 2. node-imagemagick only adds the quality option if the format is jpg
  or jpeg. Currently sending in .jpg/.jpeg
  - https://github.com/rsms/node-imagemagick/blob/master/imagemagick.js#L393
Merge pull request #3 from pdud/quality

Quality Option Fix