Encapsulation of eaddrinuse error when starting cowboy

Instead of the deeply nested error structure returned by cowboy, just
return {:error,:eaddrinuse}
Move :eaddrinuse test to cowboy/conn_test

There is already a server started on setup there, so there's less to do
to test this behaviour. Also removed the separate test for returning
{:ok,pid} on successful startup, since this is implicitly tested by the
setup in ConnTest
Merge pull request #62 from lsimoneau/eaddrinuse

Encapsulation of eaddrinuse error when starting cowboy