Add support for `equivs` kwarg on `convert_units_to`.
equivs -> equivalencies
Change default value of `relax` to True for reading.
Make default for writing WCS headers be WCSHDO_safe
Change implementation of TimeFromEpoch to support all scales

The original TimeFromEpoch (base class for 'cxcsec' and 'unix' formats)
required that the scale be TAI.  This caused errors or failure when trying
to convert to a different scale or initializing a new Time from such
a TimeFromEpoch format object.

    t = Time(100.0, format='unix')
    t2 = t.utc  # crash
    t2 = Time(t, format='iso')  # crash

New implementation does on-the-fly conversion to TAI for converting
to and from JD.  This is more expensive but makes these formats work
like any other format and not be restricted to a particular scale.
Eliminate Time._scale attr and always use Time._time.scale via Time.scale property
Add test of initializing with a TimeFromEpoch format Time object
Adds the tocdepth directive to the main index so that the 'What's New' link can be rendered as a heading, but not appear in the sidebar TOC.  Also adds a small sphinx extension to work around the bug I seem to have found in Sphinx.  Will make a bug report to Sphinx describing the issue.
Fix #596.  Store the INFO's text content properly.
Fixes the issue with pip; this will however change the default behavior of setup.py install now that we're actually using the setuptools install.  I don't think this should have any adverse affects.
Make steradians equivalent to radians ** 2.  Fixes #599.
Update the FITS unit parser to handle whitespace after the division operator.
Merge pull request #599 from mdboom/unit/steradian_radian

Make steradian and radian ** 2 units equivalent?
Merge pull request #602 from mdboom/unit/fits-parser-whitespace-handling

Improve whitespace handling in astropy.unit.Unit FITS format string parsing?
Merge pull request #596 from mdboom/vo/info-content

VOTable INFO element content not getting loaded
Merge pull request #585 from mdboom/wcs/relax_by_default

WCS: handle non-standard keywords by default
Merge pull request #449 from mdboom/unit/add_equiv_kwarg

Add support for `equivs` kwarg on `convert_units_to`.
Merge pull request #593 from taldcroft/time-from-epoch

Fix failure to convert Time objects with format='unix' or 'cxcsec'
remove sidebarsourcelink block from layout so RTD cant override
Update documentation to reflect equivs -> equivalencies