WIP: Add SimpleWebService support for /commits/:sha1/... URLs

These URLs are not in the specification yet but should probably
be added to be able to access content in a specific commit without
having to use the query API.

WIP: The top-level /commits/:sha1 URL is not implemented according
to the specification yet.
Add optional memcached support to python-consonant-server
Remove consonant.store-specific error check
WIP: Add support for testing the web API using the test suite

Missing (and thus, failing) at the moment are:

* Transactions
* Raw object properties