Creating a FAKE build

Hi,

as discussed in #116 I create a FAKE build for octokit.net.

This works as follows:

* build.cmd will download FAKE from Nuget and run the build.fsx
* Clean target: Cleans all the old stuff
* Build target: Compile the sln via MSBuild
* UnitTest target: Run xUnit
* IntegrationTest target: Run xUnit if we find the environment variables. Otherwise we print a note.
* Nuget targets: Create the packages
    * In the Reactive package we determine the version of Rx-Main and set the Octokit dependency to the current version

Open issues:

* How do you want to determine the version no.? One way is to create a ReleaseNotes.md file and FAKE can parse it (see http://fsharp.github.io/FAKE/apidocs/fake-releasenoteshelper.html)
* If this is decided we should also set the AssemblyVersion (see http://fsharp.github.io/FAKE/assemblyinfo.html)