MINOR: lua: create and register HTTP class

MINOR: lua: create and register HTTP class

This class is accessible via the TXN object. It is created only if
the attached proxy have HTTP mode. It contain all the HTTP
manipulation functions:

 - req_get_headers
 - req_del_header
 - req_rep_header
 - req_rep_value
 - req_add_header
 - req_set_header
 - req_set_method
 - req_set_path
 - req_set_query
 - req_set_uri

 - res_get_headers
 - res_del_header
 - res_rep_header
 - res_rep_value
 - res_add_header
 - res_set_header