NuGet package

This pull request adds a NuGet package spec, which lets us deploy SmartFormat to the [NuGet Gallery](http://nuget.org/). That makes it easy to use SmartFormat in projects or create other packages that depend on SmartFormat. For example, we can add SmartFormat to a project from within Visual Studio:
![SmartFormat in NuGet package manager](http://i.imgur.com/Qpm2knr.png)

Once this is done, we can deploy to NuGet in a few steps:

1. Increment the version in `src\GlobalAssemblyInfo.cs` if needed.
2. Build SmartFormat.
3. Build the package with the [NuGet command line](http://nuget.codeplex.com/releases):<br />
    `cd SmartFormat.NET\src\SmartFormat`<br />
    `nuget.exe pack`
4. Upload the resulting `SmartFormat.NET.1.*.nupkg` to [nuget.org](http://nuget.org).