stbt.draw_text: Fade out older text

`stbt.press` uses `draw_text` to draw the pressed key name on the output
video stream for 3 seconds, and I always found it very confusing when
there were several keypresses in quick succession -- is the latest
keypress at the top or at the bottom? Now it is very clear.
stbt camera calibrate: Silence pylint warning

pylint 1.3.1 was complaining:

    [W0632(unbalanced-tuple-unpacking), fit_fn] Possible unbalanced
    tuple unpacking with sequence defined at line 547 of
    scipy.optimize.minpack: left side has 2 label(s), right side has 5
    value(s)

which is clearly not true, according to the scipy documentation:
http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.optimize.curve_fit.html