Add cow_http_hd:parse_{connection,content_length,transfer_encoding}/1

Initially imported from Cowboy, then optimized.
Add cow_http:parse_{status_line,headers}/1

Imported from Cowboy and then optimized.
Update Cowlib to 0.6.0

Functions have been added to parse the status line, headers block
for HTTP, and the HTTP headers Connection, Content-Length and
Transfer-Encoding.