Use cl-lib instead of shimming, which breaks in older emacsen

Aliasing built-in (cl) functions to cl-lib versions when they are
available in older Emacs versions can seemingly lead to problems
including infinite loops during byte compilation.

Since cl-lib works with all Emacs versions supported by
multiple-cursors, just depend on this directly instead.

This commit makes the necessary changes, both to code, documentation and
package metadata.

README.md

@@ -6,13 +6,17 @@ great success and much merriment.
 
 ## Installation
 
-I highly recommend installing multiple-cursors through elpa.
+I highly recommend installing multiple-cursors through `package.el`.
 
 It's available on [marmalade](http://marmalade-repo.org/) and
 [melpa](http://melpa.milkbox.net/):
 
     M-x package-install multiple-cursors
 
+The package depends on the `cl-lib` package, so if you do not use
+`package.el` or have a recent Emacs, you would need to install that
+too: see [GNU ELPA](http://elpa.gnu.org/packages/cl-lib.html).
+
 ## Basic usage
 
 Start out with: