Add CUSTOM_HANDLERS

The idea is to make possible add new routes to the default ThumborServiceApp instead of replace it for a new one using a simple configuration.

The custom handler class must be called HttpHandler and has a static method called url_regexp which has to return the url pattern to be associated with this handler.

To enable the handler is necessary to set it on the thumbor.conf file like
CUSTOM_HANDLERS = [ 'module.where.the.handler.is' ]

Is possible to set multiple handlers, they will be added in order to the top of thumbor app.