Merge pull request #645 from adrn/sgrcoordinates-example (with changelog updates)

Conflicts:

	CHANGES.rst
Merge pull request #540 from astrofrog/constants-initial-docs

Create documentation page for astropy.constants
Merge pull request #511 from taldcroft/daophot-indef

io.ascii daophot pprint crashes on row with INDEF
Add changelog entry for #511.
Merge pull request #686 from mdboom/table/sorted-error-message

Sort the list of options in the ambiguous format error message so that t...
Merge pull request #440 from astrofrog/unit/improve-spectral-equivalencies

Add support for intensities in erg/cm^2/s in unit equivalencies
Merge pull request #691 from astrofrog/io/minor-hdf5-io-fixes

Improved opening/closing of files in HDF5 tests
Merge pull request #617 from mdboom/docs/edit-on-github-move-link

Adjust docs so "[edit this page on github]" appears next to "Page Source"