Added NuGet Packaging Script

I modified the solution and project files so they can be switched between .NET 3.5, .NET 4.0, and .NET 4.5 and added a PowerShell script to build the NuGet package.

You can run the build locally by running `build.bat -v:1.1.1 -pv:1.1.1-alpha00005` from the root directory of the project. The `-v` is for version, and `-pv` is for package version. Only the package version is compliant with [Semantic Versioning](http://semver.org/).

If you get an error indicating you don't have permission, run `Set-ExecutionPolicy RemoteSigned` within PowerShell, as described [here](http://technet.microsoft.com/en-us/library/ee176961.aspx) (you may need to also install PowerShell). 

I tested that the packages will install into projects that target .NET 3.5, .NET 4.0, and .NET 4.5.

I also tested that the script will function on MyGet. You can access the feed at https://www.myget.org/F/bobobrowse_net/. Just add this URL as a package source in NuGet Package Manager. I am still holding out hope that Alexey will maintain the permanent feed, so I have not pushed the packages to NuGet gallery just yet.

For now, I still haven't worked out how to do the versioning, because I have no idea what version of Bobo this is supposed to correspond to, and we really should be using the same version numbers, if possible.