Spatial pooler serialization ported to Cap'n Proto

See numenta/nupic#1449

- Adds SpatialPoolerProto.capnp
- Implements read and write methods in SpatialPooler class
- Adds unit test
- Adds timing test for SP serialization to prototest

Todo:

- Implement SparseMatrix serialization with Cap'n Proto so we don't have to do so in the SP