Moving contexts to OpenRasta.Testing

As a plugin developer, I frequently need to set up parts of OpenRasta
so that I can write unit tests for my extensions.

This commit moves some of the more general contexts to OpenRasta.Testing
with the aim of releasing them as a nuget package to encode best practice
for testing OR extensions.
Creating nuspec for putative package
use $ in nuspec
Merge pull request #68 from andrewchaa/PublicContexts

Moving contexts to OpenRasta.Testing