Implement djcp's tweaks

Uses secure RubyGems URL

Changed Spec::Runner to RSpec.

Gemfile.lock updated for Paperclip 3.4.1

Changed Spec::Runner to RSpec.

* since Gemfile.lock is checked in, seems like it wants to be updated following the 3.4.1 release?

Add default_url option to Quick Start example

It took me a lot of digging to figure out how to add the
`:default_url` option, but it seems a common option that many
people will need to modify. Putting it anywhere in the README
will help those who just cmd-F on the page, like me.

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.

Update link to thoughtbot logo using new public image.

Updating Cocaine to ~> 0.5.0 to get latest fixes