Add Web.RemoveFieldByInternalName and tests

Added a test that deleted the fields lefter over by earlier tests. I think this can be done some other way, not really sure what an actuall good way would be though.

Added Web.RemoveFieldByInternalName and tests for that. Notice that I'm using an ArgumentException which might not be entirely correct. Thoughts?
Should this throw some other exception? Throw on count == 0?