Add tags to nuspec

```
NuGet no longer allows editing package meta data online, a whole new package 
release is required. For the next release, we need to add the following tags to 
the nuspec:

F# fsharp unit-testing xunit nunit mbunit quotations
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 20 May 2012 at 5:25
