Add exception stack unwind support for RETURN_VALUE.

This properly implements return from try/finally block(s).

TODO: Consider if we need to do any value stack unwinding for RETURN_VALUE
case. Intuitively, this is "success" return, so value stack should be in
good shape, and unwinding shouldn't be required.
Implement default function arguments (for Python functions).

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.
Merge pull request #251 from pfalcon/return_unwind

Add exception stack unwind support for RETURN_VALUE.
Merge branch 'fun-defargs' of github.com:pfalcon/micropython into pfalcon-fun-defargs
Add mpconfigport.mk file to configure which modules to include into build.

Proof of concept, controls "ffi" module as one which requires external
dependencies.
py: mp_execute_byte_code has 2 arg arrays, for more efficient default params.
Merge pull request #254 from pfalcon/config-modules

Add mpconfigport.mk file to configure which modules to include into build
unix: libffi include path now found using pkg-config.

Hopefully this works for other people as well :)
py: Implement break/continue from an exception with finally.

Still todo: break/continue from within the finally block itself.
py: Fix emitcpy so continue is compatible with CPython.
py: Reduce code size of compiler by a bit.
py: Tidy up BINARY_OPs; negation done by special NOT bytecode.

IS_NOT and NOT_IN are now compiled to IS + NOT and IN + NOT, with a new
special NOT bytecode.
Tidy up some comments.
rt_unpack_sequence(): Support generic iterables.
Add testcase for sequence unpacking.
unix: Add basic time module (with time() and clock() functions).

Both return int so far (single-precision float doesn't have enough
bits to represent int32 precisely).
Factor out m_seq_get_fast_slice_indexes() fucntions as sequence helper.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.
Implement slicing for lists.
Implement slicing for tuples.
Implement and add tests for the id() builtin function.