Ability to build pre-release package from command line

From a powershell prompt, invoking `./build.ps1` will generate a nuget package in distribution folder.

The version number is specified in the build script `default.ps1` in the $version property.

You can optionally pass in a build number at the command line `./build.ps1 -buildNumber 2` . This will update the assembly file version:
![image](https://cloud.githubusercontent.com/assets/57436/2870137/a1d4c49e-d2af-11e3-8aaf-81bfea165dde.png)
I'd expect this to used mostly in a CI scenario, if you have one set up. I think you should be manually changing the major, minor assembly / package version yourself.

You can optionally pass in a prerelease tag `./build.ps1 -buildNumber 2 -preRelease alpha1`. This will generate:

![image](https://cloud.githubusercontent.com/assets/57436/2870153/3c8fa174-d2b1-11e3-9fce-fa19529bf863.png)
I'd expect you to run this manually when preparing to push a nuget package.