Added InvalidDescriptorException.
Added PluginManager.
Changed PluginOrder names to apply more general.
Merge remote-tracking branch 'origin/master'

Conflicts:
	AtomCore/src/main/java/com/github/atom/plugin/Plugin.java
	AtomCore/src/main/java/com/github/atom/plugin/PluginLoader.java
	AtomCore/src/main/java/com/github/atom/plugin/PluginOrder.java
Added InvalidDescriptorException.
Added PluginManager.
Changed PluginOrder names to apply more general.