Allow rbx to fail in 1.9 mode

* Based on rfc2822's commit ed05dc6b85f52ad88a83bc18e1986f3b75176740.

As per discussion in issue #660, attachments using alias interpolations
for the attachment URL mistakenly get flagged as being duplicates,
although the interpolation expands to a unique location.

This commit (partially) addresses the issue by checking the attachment path
instead. However, the problem will remain for users who use
interpolations other than :class to distinguish attachment paths for
different models.

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

This allows you to set custom file permissions on files created by
Paperclip.  Set the :override_file_permissions option to the octal file
permission you want - 0666 is the default. If you set
:override_file_permissions to false, then we will skip the explicit
chmod entirely. This makes filesystem storage a bit friendlier to filesystems
that don't support unix file permissions.

The +as_json+ method enables Attachment objects to be rendered as JSON
by Rails' JSON rendering framework. The Attachment object is not a
native ActiveRecord attribute, but it can be accessed via the +:methods+
option provided to the +to_json+ method.

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.

Updated README formatting

Fix language and typo in comments.