Create packages from the command line

Resolves #109

Done:

 - introduced Pester test runner
 - NuGet modules can be loaded in an external context

TODO:
 
 - [x] implement test suite for creating release outside of Visual Studio
 - [x] deprecate common logic inside `Create-Release` 
 - [x] document how you can use this
 - [x] validate Visual Studio usage is not impacted

Example test:

![screen shot 2013-09-08 at 1 05 18 pm](https://f.cloud.github.com/assets/359239/1102582/91705456-1833-11e3-8e58-43fbaffa66ef.png)