services.Postly: No more Mr. Nice Guy
services.Bitly: Fix STOP page parsing

Bitly switched to using the domain from the request (in tinyback's case bit.ly)
instead of always using bitly.com for the warning page.
Add documentation for tinyback.generators
service.BaseVisibliService: Add some doc. Change rate.

Add some documentaton and change the rate in case someone decides to
grab visibli/sharedby.