Add support for building with Visual Studio 2012 toolset

First removed full path to vs2010 toolset. It's not needed since they are already in PATH (assuming we first run VS Command Prompt as described in original README)

Then to build EventStore.Projections, there are two ways 
1) proposed in this request: simply add /p:PlatformToolset=v110 as MSBuild parameter
2) other way would be to conditionally set `<PlatformToolset>` in v8 vcxproj, depending on the detected version installed. I wasn't sure about that approach.