Return the default path when the attachment has no remote file

Allow either a hash or a Style object to set the headers.

S3#flush_writes was accessing the styles option without checking for a
proc. We can just use the instance's styles method to have it done for
us.

Since AWS-SDK 1.5.3 [1] the library automatically checks for credentials.
This makes it unnecessary to supply credentials when they are present
in environment variables or supplied via an IAM Instance Profile.

[1] http://aws.amazon.com/releasenotes/Ruby/0304005114813505

#expiring_url returns public_url when not using AWS to prevent exceptions when Fog Local is swapped for Fog AWS in development.

This was previously fixed in pull request #539 but no spec was added at that time, and it has since been broken.

I fixed it and added a spec.

Reverting this for now. This commit would better be in a minor release,
since it break everyone's application. Going to reopen the ticket and
mark it to be merged in next minor release.

This reverts commit b7b2b843.

* Fixed tests and functionality for new fog and removed the fixed version from the gemspec
* Set time to 3600 from the current time so expiring actually works

* Added test for checking if path gets interpolated correctly without passing
  :path or :url options
* gsub default :rails_root filesystem-like path to nothing for Fog storage,
  just like in S3 Storage.

`s3_protocol` now returns the protocol without a colon. If you need a
colon, you can pass in `true` as a second argument.

This might not be a best solution, so any better patch that does not
introduce a regression is welcome.

Fixes #921

* Add StyleTokenNotFound in Errors
* Adjust tests to make sure :style present where not currently defined

Fixes #902

Fixes #883

Re-fix of #838, #766, #911

IMPORTANT: There is a bug in the current release of Fog (1.3.1) that will
return a broken expiring URL when the bucket name is valid for a subdomain.
This is fixed per https://github.com/fog/fog/pull/857 but not released
in the gem yet. I have added currently FAILING tests for these cases that
should pass when a new Fog is released.

Make sure that we close opened files after we're done with them.