Allow overscan region to be along either axis

Closes #70
Change implementation of setting overscan axis
Merge pull request #72 from mwcraig/fix-overscan

Updated to make overscan axis an explicit argument
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 bug in sigma_clip so it does it for each pixel
Added min_value and tests for flat task
Temporary change to .travis.yml
Merge pull request #84 from mwcraig/travis-patch

Temporary change to .travis.yml
Merge pull request #82 from crawfordsm/sigma_clip_stack_bug

Fix bug in sigma_clip so it does it for each pixel
Back to regular travis configuration
Merge pull request #83 from astropy/fix_flat

Fix flat
Merge pull request #86 from mwcraig/travis-de-patch

Merging without travis build because other PRs in ccdproc have effectively already tested this