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