[close #205] Check for Rakefile before rake runs

If no Rakefile (or variant) is present we should not attempt to detect rake tasks.
Merge pull request #208 from heroku/schneems/rakefile-detect

[close #205] Check for Rakefile before rake runs
Support for turbosprockets on rails3
Remove app/assets from final slug
Clear manifest.yml
Better cache handling and app/assets exclusion from slug
Proper cache handling
Check for pre-compiled manifest before loading cache
Better order and logging
Merge pull request #1 from divoxx/master

Support for turbo-sprockets-rails3