Use 'requests' instead of urllib/urllib2 #71

As discussed in #70, requests provides a more uniform, nice, and reliable interface for the modern interwebs. One goal when refactoring should be to use requests in place of the built-ins.

As @astrofrog notes, though, we will continue to use get_readable_fileobj because of its caching and progressbar capabilities.

TODO: adapt these packages that currently import urllib and /or urllib2:

• besancon
• fermi
• irsa
• irsadust
• magpis
• ned
• nist
• nrao
• simbad
• splatalogue
• ukidss
• utils