Update and prettify README, add Travis & Coveralls badges #636
I mainly set out to let Github display the reStructuredText and add the Travis-CI/Coveralls badges, but I realized that the README is pretty out of date in general. I tried to update it to the best of my ability, but I'm not familiar enough with Paramiko to say I did it well. I'm willing to work further on this if you'd like.
For the (current) final product, see: