Add Hawkes earthquake aftershocks study example

[Mbompr]

Compiled example:

[Mbompr]

(do not merge, still problem

plot_hawkes_earthquake_example.py is not compiling:

Traceback (most recent call last):

File "/Users/travis/build/X-DataInitiative/tick/doc/sphinxext/gen_rst.py", line 885, in generate_file_rst

execfile(os.path.basename(src_file), my_globals)

File "/Users/travis/build/X-DataInitiative/tick/doc/sphinxext/gen_rst.py", line 48, in execfile

exec(code, global_vars, local_vars)

File "plot_hawkes_earthquake_example.py", line 29, in

from mpl_toolkits.basemap import Basemap

ImportError: No module named 'mpl_toolkits.basemap'
)