Minor changes

I have made a few minor changes:

* Make the README markdown.
* Add some documentation to the README for installing dependencies. I am running Ubuntu and only added Ubuntu-specific instructions for installing the packages.
* Add GPLv2+ LICENSE file.
* Decouple ./configure invocation from autogen.sh. It seems to me that the convention seems to be that autogen.sh at most invokes autoreconf and ./configure is a separate step.
* Add m4 directory that contains a .gitignore to source control so that autoreconf doesn't complain about m4 not existing.