BUG: Switch to mpl's imread which handles PIL weirdness.

Before:

    In [2]: a = pims.ImageSequence('.')[0]
    Out[2]: 
    array(<PIL.TiffImagePlugin.TiffImageFile image mode=I;16B size=1392x1040 at 0xB7330A0C>, dtype=object)

After:

    In [2]: a = pims.ImageSequence('.')[0]
    Out[2]: 
    array([[115, 117, 118, ..., 124, 123, 126],
       [118, 115, 117, ..., 126, 127, 123],
       [112, 116, 114, ..., 125, 124, 123],
       ..., 
       [104, 105, 108, ..., 110, 109, 112],
       [104, 106, 106, ..., 112, 113, 112],
       [106, 106, 106, ..., 113, 113, 113]], dtype=uint16)