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
fixes to configuration auto-generating ported from auto-generate-config branch
_generate_all_config_items adjusted to return filename
modify astropy build to make the default configuration file
fixed unnecessarily long long in setup_helpers
implement code to install the default configuration file on import
updated documentation to reflect new behavior of on-import configuration setup
removed some debug code
added encode to subprocess communicate call to support py3.x
typo fix pointed out by @mdboom
Generation of configuration error message improvements
fixed encode/decode to making error mesages helpful in 2.x and 3.x
updated setup_helpers to run in build dir - this fixes py 3.x because before the 2to3ed version did not get run
Adjusted astropy/wcs/docstrings.py to work with configuration file builder on py 3.x