Add support for backtrace on vim debug sessions

Added commands

- backtrace(b): Show backtrace and current level.
- frame (fr): Go to a level in backtrace.
- up(u): Goes up in backtrace.
- down(d): Goes down in backtrace.

to move around backtrace.

resolves #2

patch 7.4.1102

Problem:    Debugger has no stack backtrace support.
Solution:   Add "backtrace", "frame", "up" and "down" commands. (Alberto
            Fanjul, closes vim#433)