fixes a small bug that prevented changes in the comment symbol on writing

closes #790
Comparisons of Time and TimeDelta
Add tests, if only to avoid the silly bug of the previous commit
change name of parameter back to comment
Make Astropy logo linkable and redirect user to Astropy.org.
Fixed a handful of doctests that were failing; still lots more to do...
By testing out of a temp dir we lose any pytest config values specified in setup.cfg, because py.test recurses from the test dir to find the config file, not your pwd when you first start it.  So it's necessary that we copy the setup.cfg into the testing path as well.
Adds a new py.test plugin which implements a replacement/enhancement to the --doctest-modules called --doctest-plus (the name can be debated, it was just the first thing that popped into my head) which enables a special __doctest_skip__ module-level variable that makes it easy to skip testing doctest-like strings without littering the docs with '# doctest: +SKIP' directives.
Fixes several doctests in io.fits.  Most of these were doctest-like strings that never really made sense to have in the doctest format and were just non-executable examples.  A few others were made into fully working examples.  And a few more took advtange of the new __doctest_skip__ feature.
Fixed or skipped the remaining doctests that weren't working on 2.7.  Didn't put a lot of thought into most of these, but if we end up enabling doctest in the future and maybe documenting some guidelines for how to write them we can be more thoughtful about that in the future
Enable --doctest-plus by default; don't recurse into the compat subpackage
Just getting rid of this example; its outcome depends on way too many external factors; the more abstract example immediately following it gives a better illustration of how the function works (the doctest: +ELLIPSIS isn't necessary either since we add that by default now)
Added the proposed doctest_norecursedirs for outright skipping any directories where doctests that shouldn't be scanned for doctests (aside from the ones already listed in the general norecursedirs)
Fixes a few more tests in such a way that they should pass on multiple Python versions
Adds a new \ module-level global understood by the doctestplus plugin.  This skips certain tests if one or more listed modules can't be imported.  Perhaps this could later be expanded to support other kinds of environment markers (as supported by pytest.skipif) but this is sufficient for now to disable tests that blow up if certain optional dependencies (like scipy) are missing.
Because putting addopts = --doctest-plus does not work when running py.test by itself (the new options aren't picked up soon enough), here instead is a setup.cfg option to enable doctest_plus as well.
Reorganized the doctest plus plugin into a class.  Considering spinning this off into its own module.
Rebased on master and fixed a few new doctests that weren't working
This is very ugly, but necessary for the norecursedirs option to work correctly on Windows.  The problem is that py.test uses fnmatch against the pathnames, and does not normally path separators in the filename patterns.  I would consider this a bug in py.test.  In the meantime this is the simplest workaround.
For this doctest to work consistently on 64-bit and 32-bit platforms we have to specify a dtype.  I went with int32 instead of int64 because on a 32-bit Python the int64 values become Python long objects and are repr'd with the 'L' suffix.