Implement a start transaction API endpoint in Solitude (bug 987836)

- Create a Boku Transaction Form that wraps the start transaction endpoint in the Boku client
- Create an API View which wraps the form
- Add form and view tests as necessary
Merge pull request #212 from JaredKerim-Mozilla/987836

Implement a start transaction API endpoint in Solitude (bug 987836)
accept JSON in Boku transaction form
Merge pull request #216 from andymckay/fixes

use DATA not POST