Made debugging available globally, without a need to require in every namespace

[alesya-h]

It is really inconvenient having to require com.gfredericks.debug-repl in every namespace while you debugging, especially given usually one has debug-repl in their user profile, not in a project definition, so having that requires permanently in every namespace in most cases won't work.

This patch adds break!, unbreak!, unbreak!! and catch-break! to clojure.core before nrepl middleware is installed.

I chose immediate global installation rather then on-demand global installation because functions/macros will be available only to namespaces that are defined afterward unless those namespaces are manually reloaded (or unless you call (use 'clojure.core :reload) when in that namespace).

It doesn't modify any existing functions in clojure.core, only adds new ones, so it should be safe. Given debug-repl is a development tool I think it's ok to tinker with internals a little when it will lead to better development experience.