Updating to the latest casper

issue TryGhost/Ghost#1577

- adding asset helper for subdirectory support
Update activeTheme path on theme switch

fixes #1583

- the active theme path wasn't getting updated when we were switching theme
Scoping tag styling to the tag control

fixes #1309
Merge pull request #1587 from ErisDS/tag-css

Scoping tag styling to the tag control