Creating a FAKE build

Hi,

as discussed in #116 I created a FAKE build script  for octokit.net.

This works as follows:

* build.cmd will download FAKE from Nuget and start the build.fsx
* Nuget packages are restored
* Current build version is detected from the [ReleaseNotes.md](https://github.com/forki/octokit.net/blob/3c344ea80baaf7ef920b3675f8e558bb79a3fb39/ReleaseNotes.md) file
    * see http://fsharp.github.io/FAKE/apidocs/fake-releasenoteshelper.html for details
* Clean target: Cleans all the old stuff
* AssemblyVersion target: Sets the build version for the SolutionInfo.cs file
* Build target: Compile the sln via MSBuild
* UnitTest target: Run xUnit
* IntegrationTest target: Run xUnit if we find the github user in the environment variables or as build params. Otherwise we print a note.
* Nuget targets: Create the packages and fill them with version info, readme and releasenotes
    * The Octokit package has no dependencies
    * In the Reactive package we determine the version of Rx-Main and set the Octokit dependency to the current version