Temporarily remove the travis mysql-backend tests.

They're failing with some regularity and we haven't managed to dig
into the source of the problem, so for now they're just useless noise.
Refactor syncstore.js to use generic loadable backends.

The original kvstore-backend syncstore implementation is now in
lib/syncstore/kvstore.js.  Other dedicated backends to follow.

The notification-endpoint code has been factored out so that we don't
have to duplicate it across backend implementations.
Implement a mysql backend for syncstore API.

This talks to MySQL directly rather than going through the kvstore
API, allow it to operate much more efficiently.

Also includes an update to latest release of mysql client library,
which we need to support pooling and transactions.
Remove custom mysql wrapper lib.
Implement a cassandra backend for syncstore.js
Updating dev dependencies
Merge pull request #74 from mozilla/syncstore-backends

Pluggable syncstore backends
Merge pull request #76 from vladikoff/dev-deps-update

Updating dev dependencies
Merge pull request #75 from mozilla/syncstore-cassandra-backend

Syncstore cassandra backend
Enable travis tests for mysql and cassandra backends.
Rename endpoint test suite for easier distinguishability.
Merge pull request #77 from mozilla/travis-databases

Enable travis tests for mysql and cassandra backends.
Note about running different syncstore backends in the readme.