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.
Make `Angle` a subclass of `Quantity`.
Python 3.x fixes
Improves the speed of units.

- Reduces unnecessary calls to sort that were used to create unique ids.  It's faster to keep the powers and bases sorted in a canonical way all the time and then not have to re-sort on a regular basis.

- Memoize the _compose method by the unit -- this results in about 50% cache hits.

- Reduce the use of `Fraction` in calculations by converting Fractions with a power of 2 denominator.  `Fraction` instances are still used for other rationals that cannot be represented exactly by a floating point number.
Skip test_all_world2pix if scipy is not installed
Separate methods with newline
Clarify and fix logic about converting powers to floats
Merge pull request #1281 from lpsinger/iterative_all_world2pix

Add iterative wcs.all_world2pix
Alternative to #1300.  Replace update_package_files with get_package_info, which should have a more future-proof API.
Announce addition of `all_world2pix()` method

See #1281.
Merge pull request #1304 from lpsinger/iterative_wcs_whatsnew

Announce addition of `all_world2pix()` method
Use np.vectorize rather than np.nditer, for Numpy 1.5 support
Fix bounds checking.  Operations succeed if the bounds are default on one of the operands, as long as the result is within bounds.
Fix path.
Fix bug in Angle("3d")
Minor docstring fixes
Fix for io.votable.validator