Make cowlib a proper dependency

Start moving a few functions from Cowboy into cowlib.
Much improved Getting started chapter

We now describe how to build a hello world application from
start to finish, including setting up erlang.mk for building,
and using relx for generating the release. All concepts are
not explained in details of course, but we don't need to at
this point, we just want things to be working.
Implement recv timeout for SPDY
Fix wrong header sent in echo_post example
Convert the hello world example to a release
Convert the basic auth example to a release
Rename the basic_auth example to rest_basic_auth
Convert the chunked hello world example to a release
Convert the compressed response example to a release
Convert the cookie example to a release
Convert the GET echo example to a release
Convert the POST echo example to a release
Conver the error hook example to a release
Convert the EventSource example to a release
Convert the REST hello world example to a release
Add missing files to the REST basic auth example