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
Add typespecs to cowboy adapter
Fix build
Adding Plug.Test.recycle/1 to reset a test connection for use in a subsequent request.
Use Elixir GenServer instead of the erlang interface
Merge pull request #64 from rbishop/use_elixir_gen_server

Use Elixir GenServer instead of the erlang interface
Merge pull request #63 from koudelka/test-conn-recycle

Adding Plug.Test.recycle/1, resets a test connection.
Convert recycle/1 into copy_conn/2
Release v0.5.2