added a basic bitcoin gateway with an example

add a stub bitcoin backend
added bitcoin gateway, stubbed views
added basic example templates
redirect back if not ready
my html is really bad
show failure reason
cleanup
added minconf, sane defaults for settings
added a retry button
removed unused form
calculate sum based on txns
clear session if done
updated docs
added tests
Fixed the usage of the paypal integration.
Refs #47. Clarified in the documentation that the RECEIVER_EMAIL should be declared twice.
Merge wepay changes from @krotkiewicz.
Ability to use payment tokens. Changes from @dstufft
Add multiple merchant account support. Changes from @paltman.
Merge pull request #45 from agiliq/bitcoin

Added a basic bitcoin gateway with an example. Thanks @tuxcanfly.
Added the names to the CONTRIBUTORS.txt for latest contributions.
Updated the changelist with upcoming changes.
Added a reference to the bitcoin doc in the index.rst.
Added bitcoin.rst in gateways section.