Support non-byte volumes for MINC. Also fixes #259.

[rdvincent]

This pull request incorporates several related changes:

1. Updates minc2volume-viewer.js and minc2volume-viewer.py to allow a command-line argument that sets the voxel type written to the raw file. This also adds a field to the header that indicates the voxel type.
2. Updates minc.js to read this new voxel type and act accordingly.
3. Creates a scanDataRange() utility function that is used by all volume loaders to set header.voxel_min and header.voxel_max.
4. Tweaked the example's style sheet slightly, to make room for the larger voxel values.
5. Finally, this fixes issue minc2volume-viewer.py requires python >= 3.3 #259 - minc2volume-viewer.py now runs under Python 2.7.