Speed up assets in Rails by sharing `tmp` directory between deploys

The directory `/data/app/current/tmp` is now a symlink to `/data/app/shared/tmp`

Setting `shared_tmp: false` in ey.yml will disable this feature.
Merge pull request #116 from engineyard/share-tmp

Speed up assets in Rails by sharing tmp directory between deploys
Test and integrate a custom nginx.conf if provided.