Add argument for function that performs median in median_combine

Addresses #74
Merge pull request #80 from mwcraig/median-stuff

Add argument for function that performs median in median_combine
Fix docstrings as suggested by @cdeil
Add __all__ to modules
Fix astropy inter sphinx link for building docs with 0.3.1

Closes #45 but can be removed once astropy 0.4 is released
Fix intersphinx links
Update astropy intersphinx to those used in 0.4
First attempt at documentation landing page
Add documentation stubs and draft of ccddata
Wrap paragraphs
Add combiner API to index
Document Combiner clipping
Draft image combination docs
Attempt at a travis workaround

until astropy helpers is fixed...