Use async database operations where possible.

Changed the AdoController and EntityFrameworkController to inherit from AsyncApiController and use async db/EF operations where available. In theory, it should allow higher peak throughput by performing non-blocking I/O. In reality, it depends completely on the DB provider. As of writing, MySql, Npgsql and MongoDB don't support async I/O ([Mongo support is due to land in November](https://jira.mongodb.org/browse/CSHARP-138)). SqlClient does, so the results are going to be interesting if sql server is added.

Seperately, any interest in adding [NancyFX](http://www.nancyfx.org/) to the mix?