Add tests for things in utils
Use print_function
Hide directories beginning with _ from the dashboard.

Closes gh-5152
Case insensitive sorting in the dashboard.

Closes gh-5151
Don't attempt locale-aware sorting for now.

On Python 2, locale.strxfrm() can't handle unicode.
Make hidden directories configurable
Fix a couple of static analysis warnings
use on-load event to trigger resizable images

instead of timeout, which could get incorrect size information.

closes #5219
allow custom headers on all pages
Merge pull request #5169 from Carreau/allow-cors

Allow custom headers in tornado
Merge pull request #5153 from takluyver/dashboard-sorting

Dashboard sorting

closes #5151
closes #5152
make one strptime call at import of jsonutil

avoids thread safety issues (Python bug #7980)
Merge pull request #5252 from minrk/strptime-not-threadsafe

make one strptime call at import of jsonutil
Merge pull request #5223 from minrk/tiny-images

use on-load event to trigger resizable images
Don't allow edit mode up arrow to continue past index == 0
Check down too.
Get cell after first conditional
Merge pull request #5261 from jdfreder/more-dual-mode

Don't allow edit mode up arrow to continue past index == 0
Start of new callback system
Add mock to requirements for testing