Change to Markdown for README syntax

[pradyunsg]

Changed README extension to enable nice looking rendering, as well as the Coveralls and Travis Badges.

[coveralls]

Coverage remained the same when pulling 990aee3 on pradyun:master into 8113faf on enthought:master.

[coveralls]

Coverage remained the same when pulling af6ca7c on pradyun:master into 8113faf on enthought:master.

[itziakos]

The original file was using rst but had a txt extension. The README is also used in the sphinx documentation.

So a better way to make everything work together is to change txt -> rst so that github can detect the rendering engine to use

[mdickinson]

+1 for reStructuredText instead of Markdown. This is a Sphinx-related package, after all.

[itziakos]

I have updated the README.txt -> README.rst which makes the page render properly on Github. So no need to convert to markdown.