Spatial pooler serialization ported to Cap'n Proto

See numenta/nupic#1449

- Adds SpatialPoolerProto.capnp, SparseMatrixProto.capnp, and SparseBinaryMatrixProto.capnp
- Implements read and write methods in above classes
- Adds unit tests
- Adds timing test for SP serialization to prototest
Why this change?