Basic plugin architecture

Similar to http://mongoosejs.com/docs/plugins.html

Next, loopback-boot should be updated to support loading plugins from
dirs.
Minor touch-ups
Fix typo: loadPlugin(s)
Unified plugins into mixins

Mixin types: module function, module object, LDL json object.
Merge branch 'feature/plugins' of github.com:fabien/loopback-datasource-juggler into fabien-feature/plugins
Clean up the mixin processing
Merge branch 'fabien-feature/plugins'