Markdown-formatted documentation

I converted all the documentation to Github friendly markdown (http://daringfireball.net/projects/markdown) which will now make the `README`, `INSTALL` and `NEWS` files easier to read. The `INSTALL` file is also linked from the `README` file and the mention of the current release under the *Current Status* heading is linked to the `NEWS` files for easier navigation.

All three files have had the .md extension added to the end to be picked up by Github for rendering.