Fix File#exists? to actually check for existence

The current code is a no-op, because you can read back any key from the
bucket object.
Merge pull request #26 from felixbuenemann/fix-file-exists

Fix File#exists? to actually check for existence
Implement AWS::File#copy_to support

This mirrors the copy_to feature that exists in carrierwave master for
the Fog::File class. In addition it properly sets the storage class.
Allow integration tests to work with v4 regions.

For newer regions that require v4 signing, specifying the region is
mandatory or signing will fail.
Extract shared feature spec setup into spec helper
Fix AWS::File#attributes and add integration test

The method was copied from Carrierwave's Fog storage and was not
modified for the AWS SDK.
Add AWS::File#exists? integration test
Add cleanup for storing files integration test
Add AWS::File#copy_to integration test

This does not currently test if the storage class is copied, because the
AWS Ruby SDK doesn't seem to expose a method to query the storage class.