#335 Add tests for Windows 8.1
refactor: win8 and win81 test projects
 - switch to 12.0 msbuild to ensure build on VS2013 only machines
 - added explicit build dependency, win81 tests -> SL project
 - added win8 and win81 tests to unit task
 - added win8 project to release config
 - fixed win81 test project default namespace
refactor: change solution and msbuild_command to be local variables
Merge pull request #422 from adamralph/335

#335 Add tests for Windows 8.1