Update MANIFEST.in to include data files

Non-Python files need to be included in MANIFEST.in so that they are
kept track of while running `python setup.py install` and `python
setup.py test`.  This should recursively include everything in the
data directory.

MANIFEST.in

@@ -11,12 +11,12 @@ recursive-include docs *
 recursive-include licenses *
 recursive-include cextern *
 recursive-include scripts *
+recursive-include data *
 
 prune build
 prune docs/_build
 prune docs/api
 
-
 # the next few stanzas are for astropy_helpers.  It's derived from the
 # astropy_helpers/MANIFEST.in, but requires additional includes for the actual
 # package directory and egg-info.
@@ -37,5 +37,4 @@ recursive-include astropy_helpers/astropy_helpers/sphinx *
 prune astropy_helpers/build
 prune astropy_helpers/astropy_helpers/tests
 
-
 global-exclude *.pyc *.o