Adds an optional callback to CleanCSS#minify method.
Fixes #85 - adds resolving protocol @import statements.

* Rewrote inliner to process data asynchronously.
* Supports 2xx responses, redirects, errors, and timeouts.
* Supports cyclical references.
* Supports protocol-less requests (defaults to HTTP).
* Supports overriding request options - see http://nodejs.org/api/http.html#http_http_request_options_callback
* Supports timeout in ./bin/cleancss via --timeout / -t switches.
* Supports inlining local resources only without a callback.
* Supports rebasing URLs in remote @imports.
* Always triggers a callback asynchronously.
Adds inlining info in debug mode.
Fixes debug mode stats for stylesheets using `@import` statements.

* Size of inlined files were not taken into account resulting in wrong "Original size" and "Efficiency".