Add Gmail-style shortcuts

Ref #3940
- add `j`/`k` for navigating posts down / up
- `c` for opening a new post
-  `o` to open the selected post
- update keymaster dep to allow for multiple key unbinds (`key.unbind('up, j')`)
Fix GhostTab and GhostTabPane array dependencies

No issue
- TabPanes weren't finding their tab due to not listing all their dependencies in Ember.computed
Minor shortcut amends

issue #1463

- disables some of codemirrors default shortcuts to prevent them clashing
- minor cleanup of shortcut listing
Minor style fixes

no ref
- fix .featured/.unfeatured padding
- fix settings cog padding
Upgrading Casper to 1.1.0
Fix invisible tag delete icon
Update README.md

Added a link to dev.ghost.org
Merge pull request #4149 from halfdan/style-fixes

Minor style fixes
Merge pull request #4166 from JohnONolan/tag-icon-fix

Fix invisible tag delete icon
Correct broken Google webfont in editor

Fixes #4162
Merge pull request #4167 from JohnONolan/editor-fix

Correct broken Google webfont in editor
Merge pull request #4147 from ErisDS/shortcuts

Minor shortcut amends
Version bump to 0.5.2-rc2
Close mobile menu on tap.

fixes #4168
- Listen to tap event
Merge pull request #4169 from halfdan/tap-close

Close mobile menu on tap.
Upgrading Casper again
Version bump to 0.5.2
Merge pull request #4132 from novaugust/fix-tab-pane-connection

Fix GhostTab and GhostTabPane array dependencies
Abstract the allowed file types out of API

closes #1891
- Moves contentType and extension check to config
Fix relative em sizing bug