extract request variable from SubmissionPost

now used only in __init__ function
allow user to override instance and attachments directly

if overridden, spoofing a request no longer required
move extraction of params from request to outside SubmissionPost

breaks SubmissionPost API
bump version to 2.0.0 (since there are incompatible changes)
Merge pull request #76 from dimagi/extract-receiver-request

Extract request from SubmissionPost