Add NuGet Package

[SLaks]

We should be able to reference and update AppHarbor.Web.Security using NuGet.

Ideally, the package should automatically generate keys for Web.config when installed.

[friism]

We agree, but we wanted to make sure the implementation was solid before making it easy to install. If you have time, feel free to submit a PR that adds NuGet package creation - we're happy to accept contributions.