Moved constant definitions to hidden module
Fixed imports
Whitespace fixes
Upgrade to wcslib-4.16
Ignore astropy/extern directory for testing.  We don't have tests in there anyway, and this prevents the external code from being included in the PEP8 compliance report.  (I tried pep8ignore, but couldn't get that to work).
PEP8 fixes for WCS.
Make LaTeX documentation build work.  Fixes #485.
Deal with some of the low-hanging problems in the LaTeX documentation build.
Merge pull request #492 from mdboom/unit/invalid_type

give a slightly more helpful error message when an invalid unit constructor is given
Merge pull request #485 from mdboom/docs/latex-build

Building LaTeX docs fails
Merge pull request #531 from mdboom/wcs/upgrade-wcslib-4.16

Upgrade to wcslib-4.16
Merge pull request #533 from mdboom/wcs/pep8

PEP8 fixes for WCS
Make WCS transformations preserve input shape
Alternative to #518 that also reinstates broadcasting of the multiple array form.
Disabling this test on Windows for now, per #507. I can confirm that it *does* pass when the test suite is run outside of Jenkins, even on the configurations where it was failing.
Fixups after rebasing
Fixes an annoying warning that can come from pkg_resources when running
astropy.test():

```
>>>> astropy.test()
> ^[[0;33mWARNING^[[0m: Module _pytest was already imported from
> c:\python27\lib\site-packages\astropy\extern\pytest.pyc/_pytest, but c:
> \python27\lib\site-packages\pytest-2.2.1-py2.7.egg is being added to
> sys.path [_pytest.core]
> ^[[0;33mWARNING^[[0m: Module pytest was already imported from
> c:\python27\lib\site-packages\astropy\extern\pytest.pyc/pytest, but c:\p
> ython27\lib\site-packages\pytest-2.2.1-py2.7.egg is being added to sys.path
> [_pytest.core]
> ^[[0;33mWARNING^[[0m: Module py was already imported from
> c:\python27\lib\site-packages\astropy\extern\pytest.pyc\py, but c:\python27\
> lib\site-packages\py-1.4.7-py2.7.egg is being added to sys.path
> [_pytest.core]
```

This happens if you have a package installed in site-packages, but you import it from somewhere else *before* pkg_resources is imported.  That's exactly what's happening in this case, where we're using the bundled py.test to run astropy.test().  But in the process of importing py.test, pkg_resources gets imported.

When pkg_resources is first imported it builds a "working_set" of all packages available on the path (by processing .egg-info directories).  Here's it's saying "Ah, there's a package called py.test in your site-packages, but you already have a package of the same name in your sys.modules imported from elsewhere, so I'm going to skip this one".

The fix is to simply make sure pkg_resources, if available, is imported before py.test.
Merge pull request #495 from iguananaut/fits-merge

Fits merge
Merge pull request #537 from iguananaut/fix-test-warning

Fixes an annoying warning that can come from pkg_resources...
Merge pull request #523 from eteq/time-init-additions

Time initializer enhancements