Add interface to IRSA image server (IBE)

This module talks to IRSA's image server, which is currently used by WISE and PTF. The program interface is called IBE (not sure what it stands for). It supports cone searches and limited SQL-like queries to find images and catalog files for individual exposures. The system is described here:

http://www.ptf.caltech.edu/page/ibe

This is different from the existing astroquery.irsa module: that talks to a service called 'Gator' which provides access to catalogs; IBE, on the other hand, is designed for discovering data associated with given observations.