Should prob be part of #142

See also this: http://forum.espruino.com/conversations/966

Looks like timers can overlap other peripherals - so we need to be aware of that.

So I think we need:

• A list of which peripheral is currently on each pin
• A function that checks that list to see if it's possible to enter deep sleep or not
• A list of which peripherals can be on each pin (already have this in jspininfo.c)
• A function that checks which pins a peripheral is used on
• Code to shut down a given peripheral (incl. Timer Channel)

Then, every time we use a pin, we:

• Figure out if the peripheral is used on another pin, and warn if it is
• If it's a timer, check if we might be overwriting a previously set frequency
• If we were using another peripheral on our pin, shut it down first
• Finally properly initialise the peripheral on our pin

Closing - it seems nobody's been very bothered about this