warn upon pin reuse; closes #548

- `Board#register` is now an object, properly indexed by address.
- `Board#register` is of type `Object<String,Array>`, to potentially store multiple objects against the same address
- `Board#serialize` will retain the same behavior if only one object is used for an address; otherwise it calls `reduce()` for each

I couldn't figure out where to put tests for this. 
This is the commit that's most relevant: https://github.com/rwaldron/johnny-five/commit/870e6894268f959a13e9aa6123c0ea94826c0a85