Simple proof of concept implementation. Still need to decide what to do about ellipses.  Also note the warning that this could hide subtle differences in floating point representation that *could* be a bug, but also points out that there probably ought to be separate unit tests for issues like that.
Rebased the branch this is on on master.
Added code from Sympy's output checker for handling floating point
values in output.  This is similar to my previous approach but takes it
further, resolving most of the shortcomings of my original approach.  I
merged our existing OutputChecker subclass with the code from SymPy into
a new AstropyOutputChecker.

As roughly 100 lines of code were borrowed from the SymPy project I
included a copy of their license.
Added still more imprecision to this test since it was actually failing on Python 2.6
After getting some experience going through the docs and using this feature, renamed FLOAT_COMPARISON to just FLOAT_CMP for the terseness.
The regexp that searches for floating point numbers has a list of characters that may indicate the 'beginning' of a float.  I think this is a bit arbitrary in fragile, as evidenced by the fact that I had to add a left-parens to the list, as it can certainly precede a float.  Might change this later, but it's okay for now I guess.
Converted several doctests to use FLOAT_CMP
Ick. Remove a debug statement I totally left in by mistake.
More improvement to the range of contexts in which float values can appears, found through usage.
Added FLOAT_CMP to several more of the coordinates doctests, which exercise the feature very fully
Added a mention of this feature in the documentation and added a changelog entry
Merge pull request #2087 from embray/issue-2087

Parse and do float comparison of float output in doctests