Fix handling of {log_error, Code, Req, Reason} events

Although the log_error/3 event in webmachine_log has a spec that
indicates that Req is a `#wm_reqdata{}` record, the webmachine code
actually sends `{webmachine_request, #wm_reqstate{}}`.

This patch adds helper functions wm_method, wm_path, and wm_notes to
deal with the expected form. These helpers return undefined if
unexpected data is received to avoid crashing the logger and also
handle receiving a #wm_reqdata{} record.

The error and info events are logged to error_logger and NOT sent to
the request log. A line for these requests will appear there via
finish_request/acces_log event.

Tests adjusted to verify nothing sent to the request log for these
events. The error_logger output can be viewed on console when running
the tests. Integrating logging output into tests is left for another
time.

Finally, here are some notes on the log event flow in wm:

Looking at calls to gen_event:call and gen_event:sync_notify and the
specs for the calling functions in webmachine_log, I see the following [2]:

    %% @doc Notify registered log event handler of an error event.
    -spec log_error(pos_integer(), #wm_reqdata{}, term()) -> ok.
    log_error(Code, Req, Reason) ->
        gen_event:sync_notify(?EVENT_LOGGER, {log_error, Code, Req, Reason}).

However, it seems that the event logger will get called with a Req value
that has the shape {webmachine_request, #wm_reqstate{}}.

For example in webmachine_mochiweb:

    Req = webmachine:new_request(mochiweb, MochiReq)
    ...
    handle_error(500, {error, Error}, Req)

    https://github.com/basho/webmachine/blob/develop/src/webmachine_mochiweb.erl#L49
    https://github.com/basho/webmachine/blob/develop/src/webmachine_mochiweb.erl#L77

    where handle_error does:

        ErrorHandler:render_error(Code, Req, Error)

    and in default config that will be a call to
    webmachine_error_handler:render_error/3 which sure looks like it is
    expecting the tuple not #wm_reqdata{}:

    https://github.com/basho/webmachine/blob/develop/src/webmachine_error_handler.erl#L32
    https://github.com/basho/webmachine/blob/develop/src/webmachine_error_handler.erl#L93
Update README with some docs
Merge branch 'sf/log-fixes-2'