Added support for signed webhooks

See
http://help.mandrill.com/entries/23704122-Authenticating-webhook-request
s
Fixed python 3.2 unicode issues
Added byte/str compatibility for python 3
Actually handling the correct headers
Cleaned up .gitignor, PEP-8 issues and documentation
Merge pull request #39 from ProReNata/master

Added support for signed webhooks