Add iterative wcs.all_world2pix

Iterative inverse of wcs.all_pix2world. Closes #1066.
Add tolerance option for wcs.all_world2pix
Add unit test for wcs.all_world2pix
Rename pixes to pix

The name of this variable is meant to echo the name of the function,
world2pix.
Cleanup the top-level astropy namespace so it doesn't have any miscellaneous stuff
Skip test_all_world2pix if scipy is not installed
Sanitize the value of COLUMNS passed to terminal_width
Catch TypeError in case COLUMNS is empty
Fix typo in _write_with_fallback
Minor pep8 issues
Add missing import for InconsistentTableError
Add missing coma
Remove orphaned code, dims is undefined
Add missing import for _unsigned_zero
pyfits -> fits
sip1db -> self.sip1db
Separate methods with newline
WIP
WIP
Added new model tests and restructured tests