Feature/group mapping

Add group mappings, to easy remove several mapping with some common sense. Also introduce ``IInjectionMapping`` interface to hide inner method from public API.
Example:
```as3
var groupName : String = "test";
injector.map(Interface, "value").toValue(new Clazz()).toGroup(groupName);
injector.map(Interface, "singleton").toSingleton(Clazz).toGroup(groupName);
injector.map(Interface).toSingleton(Clazz);
```
.... and then just :
```as3
injector.unmapGroup(groupName);
```
All mapping that hasn't groupName still exist, but those who has already unmaped. IMHO that more handly that creating new context with all environment.