adds fallback storage driver (i.e. layer s3 over rackspace files)

[tuxpiper]

This pull request makes the following changes:

• Adds fallback driver that allows layering one storage disk over another, for incremental migrations
  • Reads: primary disk is attempted , and if the operation fails, the second disk is read
  • Writes: only primary disk is attempted
  • supports url() but not temporaryUrl()

Test checklist:

• Set up S3 bucket (with CloudFront CDN) over Rackspace files as secondary disk

  • Uploads go to S3 bucket
  • Uploads to S3 bucket are readable
  • Files in Rackspace files are successfully read and accessed

• I certify that I ran my checklist

Fixes ushahidi/platform# .

Ping @ushahidi/platform