Added SpatialPoolerExample.

Also made a small change to SpatialPoolerTest -- in https://github.com/numenta/nupic/nupic/research/spatial_pooler.py, `synPermTrimThreshold` is set to half `synPermActiveInc` during initialization (line 222: `self._synPermTrimThreshold = synPermActiveInc / 2.0`).

All test still pass.