Capture interface via Camera class

[jywarren]

Feature request

Webcam capture is currently implemented only in the parent SpectralWorkbench project, and needs porting into this library.

See specficially: https://github.com/publiclab/spectral-workbench/blob/master/app/assets/javascripts/capture.js

The interface could be:

var camera = new SpectralWorkbench.Camera();

// Spectrum would need to detect images in the constructor 
var spectrum = new SpectralWorkbench.Spectrum(camera.fetchImage({
  resolution: '800x600', // default; optional
}));

Also see the new getUserMedia camera selection API in publiclab/spectral-workbench#353

[jywarren]

I've broken out an initial step for this in #56 -- to simply port over the existing capture.js file from spectral-workbench, and hook it up. Later, we can do tighter integration!