Revised dispatcher, interval timer and batch size

Changed dispatchers so that they have a run() method for regular events
to be performed including flush(),  an 'interval' timer can also be
implemented in run() to post in batches at regular intervals for
example. The _send_data method has been split into _process_post() and
_send_post() so that the url exceptions can be reused and different
dispatchers can have custom processing. Set an absolute max number of
items per post. the default is buffersSize and can be overruled in each
dispatcher eg emoncms max of 250,.