Fix EventsDict with duplicated keys

• Fixes #68 (closed):

EventsDict([
    EventWindow('FOO', t_start='2035-01-01', t_end='2035-01-02'),
    EventWindow('FOO', t_start='2035-01-03', t_end='2035-01-04'),
])

	event	#	t_start	t_stop
0	FOO	2	2035-01-01	2035-01-04

• Add as_dict=True optional in Events find()/before()/after()/between()methods to enforceEventsDict` type return:

timeline.find('^IO', as_dict=True)

	event	#	t_start	t_stop
0	IO_TRANSIT	608	2031-01-20	2034-12-16

timeline.find('^IO').before('2031-01-21', as_dict=True)

	event	#	t_start	t_stop
0	IO_TRANSIT	-	2031-01-20	2031-01-20