Up-for-discussion: change node-imagemagick dependency to GM + more

_This includes breaking changes_. So I'm happy to maintain this as a separate
project if need be (or if this project is dead and no one responds here).

Otherwise it should probably go into a major release branch.

**Backwards-breaking**:

* Plugin configuration is totally different (see new readme) but its nice and
  makes good use of the GM api.
* Removed all of the imageMagick format stuff - not sure if it's possible with
  GM but it seems redundant to me...
* Added a few extra test cases.

Reasoning behind the change to GM is because node-imagemagick is no longer
maintained (nor is the 'new' maintained version) and GM is still very active so
definitely where it's at. In the long term I think a move to GM would be
inevitable for anyone who wants to continue using this library. I already ran
into an [edge-case issue](https://github.com/rsms/node-imagemagick/pull/74) with
node-imagick that caused the whole server to crash.