Added http method extending

Added http method extending

User can define his own response method by calling
evhttp_set_ext_method_cmp() on the struct http, or
evhttp_connection_set_ext_method_cmp() on the connection.

We expose a new stucture `evhttp_ext_method` which is passed to the
callback if it's set. So any field can be modified, with some exceptions
(in evhttp_method_):

If the cmp function is set, it has the ability to modify method, and
flags. Other fields will be ignored. Flags returned are OR'd with the
current flags.

Based on changes to the #282 from: Mark Ellzey <socket@gmail.com>