* Add a DEBUG flag to Makefile, enables: "-O0 -ggdb"

Use make V=1e make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.

This should fix issue #117

While updating the teensy Makfile, I noticed that the stm Makefile
was using calling arm-none-eabi-objcopy and size rather than
$(OBJCOPY) and $(SIZE), so I added these 2 for consistency.

Updated mconfigport.h to enable GC

* Add a TARGET definition to Makefile (default PYBOARD).
* Add support for discovery LEDs in led module.
* Add support for discovery user switch in usersw
* Add EXTI interrupt handler for discovery user switch on (PA0).
* Parameterize led and usrsw modules pins and port.
* Issue #83

* Move user switch code from main.c into a separate module (usrsw)
* Add usrsw.c to Makefile

Now much more inline with how CPython does types.

So far, only start and stop integer indexes are supported. Step is
not supported, as well as objects of arbitrary types.

mpconfig.h will automatically pull mpconfigport.h.

import works for simple cases.  Still work to do on finding the right
script, and setting globals/locals correctly when running an imported
function.

* Chane dfu.py to use Python 3 syntax to avoid dependency on Python 2.
* Update Makefile to call python instead of python2
* Fix #33

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).