Added initial tests for ScriptExecutor

Summary
* Abstracted Session and ScriptEngine into interfaces.
* Created wrapper classes for Session and ScriptEngine.
* Updated ScriptExecutor to depend on interfaces not implementations.
* Created 4 unit tests for ScriptExecutor.
* Updated namespaces of components in ScriptCs.Core project to ScriptCs.Core

Notes
* Regarding the overloads of Roslyn components' method, I thought of adding them as required. For the time being, I the ones in this commit. I can add more as required for other unit tests.

* Regarding the ExportFactory, I used it because the code originally was creating a new ScriptEngine in each execution. I'm just getting started with Roslyn, so I meant to leave the fksunctionality as it was.