Cleaning up db context creation

[kilfour]

In namespace EntityFramework.Explained.Schema; (and corresponding folder ofcourse), create an abstract class, pick a good name ;-)
This class will be used as a utility base class for all schema tests
The idea is to get rid of all the DbContext creation noise.

For now just move the function EntityFramework.Explained.Schema.Conventions.GetReader in there, making it protected ofcourse, and maybe name it ReadFrom