Updated plaid client to use the v2.2 beta API internally, while keeping the external API the same.

Client changes:

* Changed to use latest HTTP URL endpoints
* Using the correct HTTP method for each endpoint
* Setting only the correct query parameters for each endpoint
* Stopped lifting special 'login' parameter out of 'options' and into the main param list
* Switched to using form POST params instead of query string params, matching the docs
* Treating any 2xx response code as success, to support the 201 code returned for MFA
* Fixed broken check for presence of mfa field
* Removed unused '_handleMFA' function from client library
* Added a new function for 'get_entity'

Unit test changes:

* Stopped taking in username, password, type, and email from environment - now using the standard 'plaid_test', 'plaid_good', from the docs.
* Added a unit test for each of the 5 account types (amex, bofa, chase, citi, wells)
* Stopped asking for manual entry of MFA credential during unit tests. Unit tests should run to completion without user input.
* Added a unit test for locked accounts using 'plaid_locked' password.
* Updated existing tests to look for new messages.