extract request variable from SubmissionPost

now used only in __init__
allow all request-based variables to be passed in explicitly

if request is also passed in, the explicit value overrides the request value
if all required values are passed in, request need not be

this makes it possible to call directly without spoofing a request
Merge branch 'master' into extract-receiver-request
move extraction of params from request to outside SubmissionPost

breaks SubmissionPost API
always add last_sync_token and openrosa_headers even when None
update couchforms dependency
Merge pull request #12 from dimagi/extract-receiver-request

Extract request from SubmissionPost