Add String#remove{,!}

I kinda hesitate whether this one is really useful, but I like it. It is a shortcut for the common `gsub(pattern, '')` with a bit more explicit name to it. There is [`String#delete`](http://ruby-doc.org/core-2.0.0/String.html#method-i-delete) in the standard library, but it's semantics are quite different.

Anyway, I like the expressiveness of it, its already in [Active Support](https://github.com/rails/docrails/commit/5da23a3f921f0a4a3139495d2779ab0d3bd4cb5f) and maybe you'll like it too.