Removes check for potential clashes in attachment paths, per discussion
in #1143.

As of 26f4d409, the default path settings no longer lead to this problem,
and the checker is more likely to generate false than true positives as
it cannot fully expand interpolations.

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

The ":" is a special character in file paths passed to ImageMagick's
"identify" command. Even though Cocaine (and everything else in
paperclip) escapes special characters correctly, we need to strip it out
of filenames to ensure identify (and probably other ImageMagick
commands) can run correctly.

This reverts commit 68aea16f.

This reverts commit 394ea1f6.