Change Caching Method to Use md5 of File Contents Instead of Modification Times

[crankeye]

Changes

• Checks the md5 of the file contents instead of the file modification times when checking to see if a file should be reprocessed.

  About

• This brings it more in line with the rails asset pipeline: http://guides.rubyonrails.org/asset_pipeline.html#what-is-fingerprinting-and-why-should-i-care-questionmark
• Also fixes an issue in a multiple server environment. When deploying to multiple servers the file modification times are slightly different. This causes the md5 of the files to be slightly different on Server A and and Server B. When load balancing between the servers sometimes the first request will be routed to Server A, then the second request for the css file we be routed to Server B which will 404. If we use an md5 of the file contents this will no longer be an issue because the hash will be exactly the same.