Added missing MANIFEST.in
Removed (unused) mocksignature import

mocksignature no longer appears to be included with recent versions of Mock
Added tox.ini for easier testing
Tidied up trailing whitespace
Python 3 compatibility
Forward original headers when processing includes

ESI originated requests now include the original cookies and
authorization headers. This allows us to use ESIs to include
user specific or restricted access information on pages.

Rather than forwarding all headers from the original request, this
filters out a whitelist of headers that are desirable to forward. For
example if while processing a POST request, the 'Content-Length' header
were to be forwarded on an ESI subrequest, it would cause the remote
server to wait for further content, and the request will timeout.