ENH: new api method: add_event

This is another version of a time/event management api.

This version allows users to schedule functions to be called with (context, data) args at frequencies other than the backtest frequency. Right now the functions are called before the regular handle_data routine, but after could make sense, it would allow people to set context variables.

Example workflow
```
def func_a(context, data):
    # stuff
 
def func_b(context, data):
    # other stuff ...

def func_c(context, data):
    # ...
 
def initialize(context):

    # Set functions to be called every 20 min
    add_event(freq='20T', functions=[func_a, func_b])

    # For once/day events, specify a 'when' function
    add_event(time_func=BeforeClose(minutes=5), functions=[func_c])
 
def handle_data(context, data):
    # Nothing to be done
    pass
```
@humdings can you give an example where you'd want to set a context variable in `handle_data` that's required for a callback to work correctly?  In particular, is there a case where you couldn't set the same value during `initialize`?  It seems like we want to discourage the practice of updating shared context variables using this API, for the reasons pointed out by @twiecki in his comments here https://github.com/quantopian/zipline/pull/386#issuecomment-54370847.