bit of work

Closes #602

So instead of having:

    http://s3.amazonaws.com/paperclip/var/www/foo.com/current/avatars/1/original/foo.jpg

because default path has the :rails_root, now you'd have:

    http://s3.amazonaws.com/paperclip/avatars/1/original/foo.jpg

This shouldn't affect your application because you'd normally set the path to some string.

Paperclip::InterpolatedString will ensure that we're not double escape the string, which was the cause of the escaped filename problem in #path method.

Added comments and add explicit ordering to this method, which makes the result deterministic and resumeable should an error occur mid-way. 

As values in set are strings - check is unnecessary.

Saving instances causes memory leaks in development mode in rails.

It's Paperclip's responsibility to escape special characters from the URL to make sure that it's comply with standard.

Closes #577, Closes #563, and reverse my judgement on #482.

It was working fine for me locally

fixes #588 . Attempt to log warning message when 2 models use same attachment name with default url interpolation

There was an issue on assignment another S3 attachment object to another object whereas calling #to_file on S3 attachment will create a Tempfile, which would then having a different file name than original version.

By caching the filename first before calling #to_file, we're ensuring that filename is the original one.

Fixes #9