Stop-gap support for neural networks

Follow on from pull #62, this patch implements very basic support for neural networks.

* Weights are represented explicitly within `Network`.
* `MultiLayerNet` uses `Network` and defines back-propagation in terms of the nodes which are actually connected. 
* Introduces support for storing `BinaryAttributes` efficiently.
* Introduces `FloatConvertFilter` and some new tests.

[Documentation.](https://godoc.org/github.com/Sentimentron/golearn)