Enable parallel execution on NUnitLite under .NET desktop builds
Merge branch 'issue-334' into issue-333
Fix error under compact framework
Remove appveyor.yml changes added in error
Merge pull request #339 from nunit/issue-333

Issue 333, unties parallel execution from the NUNITLITE define, instead using a PARALLEL define and removes the distinction between the NUnitTestAssemblyRunner and NUnitLiteTestAssemblyRunner