stop storing items in tmp/ directory, only copy bundled ember when no over-ride present 

Every time Rails is initialized, ember-rails copies ember & ember-data from the local gem to tmp/ember-rails. When a user runs the ember-rails updater, versions are stored in /vendor/assets. If the user updates their ember gems instead, updated ember copies are placed in tmp/.

Changes:
1) Move the /tmp folder to a /bundled in the ember assets folder. This serves the same purpose, but means the gem only ever copies ember to one spot. (No more rake tmp:clear / updated versions in potentially 2 places)
2) Make sure there is no updated/over-ride copy of ember, and only copy the if there is none (no duplicates).

Fixes #343 & possibly issues from #345.