Move README file to reStructuredText

The PyPI page is difficult to read because PyPI doesn't understand markdown:
![selection_067](https://f.cloud.github.com/assets/285352/1151478/923a48fa-1efb-11e3-8acd-eaeed8220d4e.png)

The README file could also benefit from syntax highlighting (which works on Github and PyPI).

Before (no syntax highlighting):
![selection_068](https://f.cloud.github.com/assets/285352/1151491/b4718334-1efb-11e3-86ec-979efc668215.png)

After (syntax highlighting):
![selection_069](https://f.cloud.github.com/assets/285352/1151500/c31c0850-1efb-11e3-9138-0b4092249f19.png)

As you can see from the diff, rST isn't more difficult to read than markdown.