Added wcs module with utility function for determining the order of a 
PV matrix.
WIP: WCS xy2sky function
Updated wcs.get_order of PV matrix to take into account PV?_0 terms.
Added some realistic test data for xy2sky
Added Fortran source code for xy2sky along with a test Fortran program
for running it on some test data.  This will be used as expected data in 
Python test cases.
xy2sky implemented and tested against Fortran output.  Values are the 
same up to a delta of 0.000005
Removed get_order function as nord is a parameter of xy2sky
Basic Makefile with target testxy for building test_xy2sky.f and wcs.f,
running the test code, and then cleaning things up.
Minor cleanup
Fortran subroutine sky2xy plus some test code to run it for test data.
Added target to Makefile.
almost_equal matcher for asserting floating point numbers have some 
number of significant figures in common.
sky2xy Python implementation
Allow the maximum iterations for sky2xy to be specified, but use a default
value.
Merge branch 'wcs' into gui