Readers

Based on the astropy I/O registry I wrote several reader for file formats that I had to read in the past few weeks.
The IUE Reader is the most complete, the redch Reader for REDUCE (a popular IDL echelle data reduction packege) is only missing a few keywords.
The Reader for IRAF multispec might be superceeded by a more compete implementation of the WCS standard, once the models are merged into astropy. 
Still, I believe we should get going on the Spectrum1D class at some point.
Tests for those reader are included, but not complete (I don't want to invest too much time in tests for this code, since I don't know if the maintainer / coordinators accept this contribution or have other plans already).