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Initial support for Teensy 3.1

* Fix LED pin enum, first one should start at 1
* Fix LED initialization typo

Implements list.reverse; fixes issue #66.

List insert. Fixes issue #61.

Fix off-by-one in non-default values of index's 2nd and 3rd arguments.

Convert many object types structs to use C99 tagged initializer syntax.

Add Initial Support for STM32F4DISCOVERY Board

Make qstr_init reinitialize last_pool.

This causes the pool to get reinitialized properly on a soft-reset.

* 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 into a separate module

Implements str iterator

list.index fails on an edge case.

Implements list.index. Fixes issue #57.

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