Implement sys.path support

This implement sys.path support - handling on ``__import__()``'s side, and initialization of sys.path value for unix port. Adding needed changes for stm should be more or less trivial based on the unix code. There may be opportunities for refactoring and reusing some code. But I just wanted to put up this for review.

And some comments about patches: yeah, nobody likes to do string processing in C, especially if without dynamic memory alloc. But someone needs to overcome that and try ;-).