Merge pull request #307 from franckstauffer/imagick

[NEW] Support for imagick based resizing
[UPDATE] Provide an interface to manage plugins
[UPDATE] Improve Plugins interface
[UPDATE] New Unit tests for Plugins
[UPDATE] New Unit Tests for User and Permissions
Merge pull request #308 from renaudJennyRBS/plugins

Provide an interface to manage plugins, and add some Unit Tests
[FIX] Some improvements in thumbnail generation