Expose constants defined in calvinsys to Calvin scripts

[fswensson]

It would be nice to be able to use constants defined in an object in calvinsys from an Calvin script.

Example:
calvinsys/io/gpio_handler defines the constants PULLUP and PULLDOWN for enabling internal pull up/down resistors, these are then accessible from an Calvin script:

gpio : io.GPIOReader(pull=calvinsys.io.gpio_handler.PULLUP)

[persquare]

What about @io.gpio_handler.PULLUP using @ to signify an "address" to a value? Could be used as is, or in via a define PULLUP = @io.gpio_handler.PULLUP.

[fswensson]

Looks good!