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.


Don't need this line if the `@return` field is detailed (see docstrings of other `mmGetMetric...` functions).