[WIP] Support for SQL Server Compact

An implementation that uses SQL Server Compact Edition as a backing store. 

I'm trying to keep it similar to how SQLite works, and I don't want to pull in EntityFramework for what's basically just mapping two tables - so I'm doing raw ADO.NET here instead.

Heads up: you can only specify a file path which is <= 128 characters. Go SQL Server CE!

 - [x] the basics around caching and retrieving values
 - [ ] implement all the *Object APIs
 - [ ] encryption support
 - [ ] support for specifying `File Mode=Shared Read;` and `File Mode=Read Only`; in connection strings
 - [ ] support for specifying `Max Buffer Size=1024;` (kB) to control when changes are flushed to disk
 - [ ] move tests into main project