Reduce requirements for core package

[aforren1]

I think numpy and future the only "real" dependencies -- everything else is either distributed with python by default, or is optional (only required if you want to use the particular device). The question might be whether we lump all of the optional dependencies together, i.e.

...
    extras_require={
        'devices':  ['hidapi', 'pyserial', 'keyboard']
    }
...

or

...
    extras_require={
        'hand': ['hidapi'],
        'birds': ['pyserial']
    }
...

or offer both options.

[aforren1]

Closed by #40