readme reformatting and edits

originally i just wanted to fix the crazy long line; i may have gone a bit overboard on the edits...  i added some hard line breaks, so it should look slightly better on PyPI, too.  you can see what it looks like rendered here: https://github.com/dystedium/pyjwt/tree/readme_change

as a side note, does the library still only implement draft 1 of the spec?  [it's up to 15 now](http://tools.ietf.org/html/draft-ietf-oauth-json-web-token-15).