Remove Guice AOP and dependency injection details from commands.

Remove the need for all commands to know about the AOP framework
being used. All of them are given an instance of org.geogit.api.Injector
from which to have direct access to the repository objects
like Object/Staging/Graph/Config/RefDatabase, WorkingTree, Index, etc

The API is hence made clearer and more succint, leaving the actual
dependency injection details to be handled only by the org.geogit.di
package.