Split database methods into two namespaces

The methods have been split into two namespaces describing schematic
changes and data changes.
Removed unused using directives
Added support and tests for alter table queries
Added support and tests for dropping columns
#20 Added migration database example
Fix + test for migration cancelling endless loop
Database util extensions now maintain IEnumerable

We didn't need the list, for the most part it's just checked *for* and
object existence
#20 Migrations now check for db errors
Simplified the commands example

I reworked how referenced are stored for bubbling/tunneling commands but
the example wasn't updated.