Create SP Mixin #64

@chetan51 @subutai 

I have created a mixin monitor class for the spatial pooler. Currently the following metrics are collected
1. Indices of active column
2. Active duty cycle of each column
3. Total number of connections.

Let me know if more metrics are desired to be collected.


Just use a regular Python `list` instead of `numpy.array`, so that we can make a `Metric` out of this data.

Also, I suggest adding a method `mmGetMetricDutyCycles` (for a similar example, see https://github.com/numenta/nupic/blob/master/nupic/research/monitor_mixin/temporal_memory_monitor_mixin.py#L143).