Add AstropyBuildPy, which builds the default configuration even if build_ext doesn't run

closes #719
Prevent configuration file generation in build_py if build_ext will be run
Merge pull request #9 from astrofrog/build-py-generate-config-fix

Prevent configuration file generation in build_py if build_ext will be run
Updated docs to including coordinate string formatting
Update list of author names using the result of `git log --format='%aN' | sort -u`
Changed default config generation to use Distribution.is_pure to know when to run
Missing space typo pointed out by @mdboom
Include 'hr' as an alias for 'hour'
made travis build_sphinx command not do intersphinx
Removed value setter to make Quantity immutable

Closes #761
Updated tests to check that quantity is immutable
Add Nadia Dencheva for her contributions to pywcs.
Merge pull request #753 from eteq/build-py-generate-config

Fix #719 by adding build_py config generation
Merge pull request #763 from eteq/no-intersphinx-in-travis

made travis build_sphinx command not do intersphinx
Merge pull request #758 from eteq/coordinate-string-formatting-docs

Updated docs to including coordinate string formatting
Merge pull request #757 from mdboom/unit/hr_as_hour

Change/allow  hour unit to be 'hr'
Merge pull request #749 from mdboom/docs/credits

Update list of contributors in docs
Merge pull request #762 from eteq/make-quantity-immutable

Make quantity immutable
Fixes the issue reported in #769, and a few other uses of check_output which won't work on Python 2.6
Refactor astropy.constants API