Added as_iterator method, refactored base resource types
Renamed Resource -> GenericResource
Merged in refactoring (pull request #43)

Refactoring
as iterator commented
add messages resource
add test for messages
change sender instance of batch send method according to pull request
add message property to MessageEvent,
add tests fro MessageEvent
Added states to CAUSES enum
make list of errors with Message instances and error_messages,
add filling up messages that created in batch method
Merged in APT-21 (pull request #44)

add messages resource
Merged in causes (pull request #45)

Added states to CAUSES enum