Rebased the Portable branch on master and squashed commits.
Fixing the portable build after rebasing off master
Removed the Mixed Platforms platform that Visual Studio keeps re-adding
Merged in latest from master
Merged in latest from master
Updated the build scripts to build portable
Removed portable from the Travis build and added portable projects to NUnit.Linux.sln
Removed portable from the linux solution
Merge pull request #313 from nunit/issue-261

Issue 261 - NUnit Portable - Appveyor error is unrelated, so merging.