custom MyGet build instructions

This MyGet.bat contains custom build instructions and will execute the following flow:

1. restore NuGet packages for the test project
2. build the test project (and the referenced PUrify.csproj, this avoids building it twice in subsequent msbuild calls)
3. run unit tests
4. create the PUrify.nupkg and PUrify.symbols.nupkg
5. host the package on your MyGet feed
6. if the build source is configured to push the symbols package to symbolsource, myget will do that for you as well

Happy 2014! :)