Fix loop handler keepalive race condition

Previously if a loop handler received the timeout message from a
previous request on the same connection the socket would be set to
{active, once} incorrectly - when a socket packet was already in the
message queue. This second packet would not be added to the buffer
before a Handler:info/3 call if a user message was in the message
queue before both socket packets.
Fix tohexu/1 and tohexl/1

tohexu(16) or tohexl(16) will now crash, instead of evaluating to $G or $g.
manual: Add note about cowlib
Update erlang.mk
Update erlang.mk again
Remove the confusing doc folder

All documentation is in README.md, guide/ or manual/ now.
Merge branch 'master' of git://github.com/KlausTrainer/cowboy
Merge branch 'to_hex_fix' of git://github.com/matthias-endler/cowboy
Merge branch 'fix/keepalive_loop_stream_recv' of git://github.com/fishcakez/cowboy
example: fix typo in doc http to https

ssl_hello_world_example uses https instead of http.
Fix echo_get/README.md
Fix the SPDY suite for R15
Merge branch 'patch-1' of git://github.com/edgurgel/cowboy
Fix a small typo in a test
Nicer error when NPN isn't available
Merge branch 'typofix' of git://github.com/Etsukata/cowboy
Clarify functions that can only be called once
Fix typespecs for cowboy_req:binding/{2,3} and :bindings/1
Now testing on R15B01+ and maint and master git branches
README.md: tweak grammer