Protect against function/method confusion

[profburke]

If you accidentally call a blink method as a function, i.e. if you do something like the following:

local d = blink.open(0)
d.set(128, 255, 100) -- should be d:set(128, 255, 100)

you get a seg fault.

Need to figure out how to detect this and give an error, rather than a crash.