Create an additional property use_salesforce_sandbox and change
the URL for web2lead form if using sandbox.

Previous versions checked for specific HTML in response string
from SF web2lead form, which has since changed.  Validate based
on presence or not of 'is-processed' response header instead.
Intuitively, 'is-processed' will be sent if there is an error.

Product should really be updated to use a solid API insted of old
web2lead form.

Update tests accordingly.

PEP8 fixes.

Update TODO, CONTRIBUTORS