Implement sys.path support

Fixes Issue #261.

This mirrors CPython behavior and makes possible to run scripts which
import other modules not from script's directory.

If it's not available, "~/.micropython/lib:/usr/lib/micropython" is used
as a fallback.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.

Initial support for Netduino

Similar to mp_obj_new_tuple().

This also fixes up the IRQ for the PYBOARD4 USERSW
although I was unable to test that functionality.

Only works on modules and class instances.

Structure is back to expected 16 bytes.

Native emitter has been broken since stack order has changed from
reverse to standard.  This fix gets it partially working.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

Both return int so far (single-precision float doesn't have enough
bits to represent int32 precisely).

IS_NOT and NOT_IN are now compiled to IS + NOT and IN + NOT, with a new
special NOT bytecode.

Still todo: break/continue from within the finally block itself.

Hopefully this works for other people as well :)