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.


As Subutai pointed out [here](https://github.com/numenta/nupic.research/issues/64). The SP's duty cycle is only during learning and is a rolling average. It is desired to have something that's just a simple average over all patterns.

I think the rationale behind the trace data structure is the minimal amount of data you collected on every cycle. Since the duty cycle can be easily derived from the activeColumn trace, I feel it should be a summary over the entire history.

I will rename the method as mmGetDataDutyCycles then.