Use erlang.mk to provide make targets

Included version is from commit 90cba7b8 of
https://github.com/extend/erlang.mk.
Add initial support for preflight requests

This follows the steps in section 6.2 of the specification.  Steps 1
and 2 of both section 6.1 and 6.2 have been combined as intial sanity
checking steps before selecting a processing model.
Add example application
Merge pull request #1 from danielwhite/preflight

Add initial support for preflight requests
Merge pull request #2 from danielwhite/example

An example application
Merge branch 'master' into cowboy0.8.3

Conflicts:
	rebar.config