message-handler: Add NgxMessageHandler

This adds an NgxMessageHandler, that uses ngx_log_error to output messages. It
is heavily based on apache_message_handler from mod_pagespeed.

It falls back to an internal GoogleMessageHandler if the required initialisation
has not been performed yet.

It can also install a crash handler, that will try to log a backtrace upon
receiving certain signals. In preparation of us porting /ngx_pagespeed_messages,
it also keeps the last few messages in a SharedCircularBuffer

(Squash-merge of Otto's #163)