Add linked header in sphinx source for bridgedb.captcha.
Change PTTBB link to TBB-3.6-beta-1 download page.

  * CHANGE link text to call it "Tor Browser Bundle", due to PTTBB and
    TBB being unified into the same bundle.
PEP8 for EmailServer module docstring.
PEP8 cleanup import lines in EmailServer.
PEP8 whitespace fixes for EmailServer.
Import `defer` rather than `Deferred` in EmailServer.
Remove unused import ConnectionRefusedError in EmailServer.
Use `from` import syntax for twisted.mail.smtp in EmailServer.