Add support for removing objects from a Realm directly from the RealmObject

This also clarifies the error message when a deleted object is being 
operated on.
Add documentation to the deleteFromRealm method
Update the changelog
Merge remote-tracking branch 'origin/master' into ez-remove-from-realm

Conflicts:
	realm/src/main/java/io/realm/RealmObject.java
Merge pull request #522 from realm/ez-remove-from-realm

Add support for removing objects from a Realm directly from the RealmObject