fix for #378 auto-package and version

Right now the version number must be updated in several places in order to publish the nuget package, which has led to outdated packages on NuGet (see #378).

I'm trying to make this easier by 

- [x] Updating `SharedAssemblyInfo.cs` to define the version, file version and informational versions based on a single value and have this value used throughout the solution.
- [x] Adding an `XmlUpdate` task to the RestSharp project that will update the version element in restsharp.nuspec.
- [x] Having `package.cmd` trigger this task to update the version number before executing nuget pack. 
- [ ] Pulling the AssemblyInformationalVersion from the assembly

Right now I'm pulling the AssemblyVersion rather than the AssemblyInformationalVersion.

@Haacked, know anyone who could help?