Add propertyName to the attachment's defaultUrl

- allows multiple attachments with the same styles

closes #1
Update conditional '/' prefix
Merge pull request #44 from shake-apps/master

Add propertyName to the attachment's defaultUrl
Use custom imagemagick.
Use git url.
Added the filenameId attribute.

filenameId allows the user to customize the filename.
Before: /dir/objectId-original.jpg
Now: /dir/<custom-model-attribute>-original.jog
Merge branch 'wedis'

Conflicts:
	lib/attachments.js
Add gm dependency.
Tabs to spaces.
Setup testing.
Formatting.
Watch script.
Bootstrap - make it work with watch.
Expose providersRegistry.
Ensure that the provider actually writes the new file.
Thumbnail should be smaller.
Add test for resampling.
Reorganise specs.
README first - tweak how attachment configuration looks.
Tweak the spec.