Use a bootstrapper

This will allows use to completely remove nuget.exe.

/cc @agross @ilkerde 

7KB bootstrapper vs. 1,58 MB NuGet.exe

It downloads the latest version from github releases (only if paket.exe doesn't exists or it is outdated)

![boot](https://cloud.githubusercontent.com/assets/57396/4324802/147f3fa4-3f5e-11e4-825d-1672f147dc1d.gif)

Should wrap this in a using block, too.