Support JSON manifests and manifests with randomized filenames

Rails 4 now writes a JSON asset manifest file with a randomized filename.  This commit expands the ```deploy:assets``` tasks to support it.  It is completely compatible with YAML manifests as well as manifests that do not have a randomized filename.  

Rollbacks are still broken, though, because there doesn't appear to be an ```assets:precompile:nondigest``` task or equivalent in Rails 4.  I'll have to look into that further.

Fixes #362