Use erl -eval instead of rebar for generating edoc documentation
Stop using rebar in the Makefile

First and foremost: yes, you can still use Cowboy as a rebar dependency.
This commit only removes the use of rebar when *developing* Cowboy, not
when *using* Cowboy.

Over the past two years I went from very happy with rebar to unsatisfied
and most recently found it counter productive in many ways, from having
insane default configuration to various unefficient operations. The earlier
reversal from 'rebar ct' to 'ct_run' made my workflow much more natural,
as I always needed to look at 'logs/raw.log' to find out what was wrong,
anyway. Why not let 'ct_run' output it directly instead? Removing rebar
made my life easier.

If you wonder why I don't patch rebar, there's two reasons. First is that
the direction taken by rebar isn't compatible with my views, and this
would be a huge fight to steer it in another direction. I got other,
more important fights to make. Second is that I'd rather patch OTP so
that everyone benefits from it, not just users of rebar.

Anyway this isn't my personal blog so I will stop babbling here. There's
a few important things to note relative to this commit:

 *  You don't need rebar to work on Cowboy anymore
 *  The eunit tests are now ran through common_test

Ping me if it doesn't work out for you.