Parse 'Connection' headers as a list of tokens

Replaces the 'Connection' interpretation in cowboy_http_protocol
from raw value to the parsed value, looking for a single token
matching close/keep-alive instead of the whole raw value (which
could contain more than one token, for example with Firefox 6+
using websocket).

Introduce the functions cowboy_http_req:parse_header/2 and /3
to semantically parse the header values and return a proper
Erlang term.

include/http.hrl

@@ -58,6 +58,7 @@
 	raw_qs     = undefined :: undefined | binary(),
 	bindings   = undefined :: undefined | cowboy_dispatcher:bindings(),
 	headers    = []        :: http_headers(),
+	p_headers  = []        :: [any()], %% @todo Improve those specs.
 	cookies    = undefined :: undefined | http_cookies(),
 
 	%% Request body.