copy in a version of the original django div.action

it's not an exact copy of what's in django 1.4, but it's really close. I
just added some documentation and some copy changes. I pulled the
markup from my browser window. I'm not sure where the original django
code lives.
add some bootstrap styling to the buttons
use smaller buttons to de-emphasize
move buttons into actions toolbar
Merge pull request #33 from texastribune/fix-missing-select-across

Fix missing select across logic in change_list