Navigation and variable view.

* Navigate using step over, step into, step return.
* View variables in current stack.
* Register debug event set listener to handle breakpoints, stepping
  operations.
* Fixed checkstyle warnings.
* Added a documentation file debug.rst

TODO
Functionalities
* Viewing variables that are objects.

Bugs
* The temp variable viewing window position changes to top after a vim
  command is sent as part of stepping through.

Testing:
Verified step over, step in, step return, get variables.