vm: Introduce structure for exception stack entry, record entry type.

Also, handle SETUP_FINALLY opcode.
Update VM stacks comments.
Add basic try-finally testcase.

It's mildly suprising these work without further changes to exception
handling code (the only change required was to handle SETUP_FINALLY).
Merge pull request #246 from pfalcon/exc_stack_entry

vm: Introduce structure for exception stack entry, record entry type.