Simplified most acceptance tests, which also dramatically improved performance

Pre-caching environment and test framework strings to reduce the time of AssemblyResult.ToXml()

Upgraded projects to 4.0
Updated APIs which previously needed security demands to now use [SecurityCritical] and [SecuritySafeCritical] as appropriate
Updated xUnit.net version number to 2.0

Update xunit.extensions.nuspec to use an exact version dependency
Update to support new version dependency in RegEx replace, as well as push libraries into lib\net45