Move the RedcarpetCompat class to its own file

Since the RedCloth API layer is now deprecated and will be removed in
4.0, let's move this class to its own file to make the main one more
clear.

We would have to require "redcarpet/compat" to be fully backward
compatible but the RedcarpetCompat class shouldn't be use anyway. It
is just a convenient way not to override the `Markdown` one if it
already exist.
Specify that the RedCloth API layer is deprecated
Remove the RedCloth layer's section from the README
Merge pull request #401 from vmg/deprecate-compat-layer

Deprecate the RedCloth API compatibility layer