Documentation update and tools

Uses the F# Formatting documentation tools to generate nice documentation for the R provider. A local copy of the documentation [is available here](http://tpetricek.github.io/FSharp.RProvider//index.html). The content is copied from the R provider wiki pages.

To update the documentation, open `Tools/build.fsx` and run everything (the `build()` function at the end). This creates a documentation from `*.fsx` and `*.md` files located under `Documentation` and stores the resulting HTML pages in `Documentation/output` (to host this on github, I copied the content of this directory to a `gh-pages` branch..)

The Markdown part should be fairly obvious. More interestingly, see [tutorial.fsx](https://github.com/tpetricek/FSharp.RProvider/blob/master/Documentation/tutorial.fsx), which now includes some additional comments and is used to generate a nice documentation page.