added unit examples & link to astropy.coordinates docs.
added hard returns to line wrap.
clarified sexagesimal.

apparently, sexidecimal is too risque. http://webcache.googleusercontent.com/search?q=cache:1mNfB-oWXBwJ:www.exampleproblems.com/wiki/index.php/Hexadecimal+&cd=10&hl=en&ct=clnk&gl=us&client=firefox-a
Merge pull request #2105 from kelle/master

added unit examples & link to astropy.coordinates docs.