begin working on database router
Merge branch 'feature/bulk-messaging-api' into feature/database-router

Conflicts:
	tests/default.py
stub out sending functionality
mark transmissions status in receive(); add receive tests
add more tests
use bulk create with transmissions
chunk transmissions into managable sizes
add comments and docstring
clean up admin a bit
pass queryset into IncomingMessage object
add direction column to message list admin
use send API when sending responses to ensure settings.RAPIDSMS_ROUTER is used
add message to transmission list; search by message text as well
convert transmission ids to list before sending to task
add indexes to commonly filtered fields
enhance connection admin
move response processing to base router
store responses and context dicts so we don't re-create OutgoingMessages later; update base router tests to UnitTest; add tests for new_incoming/outgoing message functions on base router
rename in_reply_to to in_response_to; add test to make sure DatabaseRouter sets db in_response_to field
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