Auto update debug status view.

* When debug events get fired, the status window is updated using
  VimClient.refreshDebugStatus which in turn triggers a VIM command to
  request for the debug status. So the control flow is:
  Java->VIM->Java->VIM.
* DebuggerContextManager: New class to keep track of debugging sessions.
  With this change, DebuggerContext is created and destroyed for each
  session instead of getting reinitialized. It makes the code cleaner
  and gets rid off race conditions due to re-initialization.
* debug.vim: Moved the :JavaDebugStatus outside of ftplugin into plugin
  dir. This is necessary because this command is invoked from an
  external VIM command. If at this time, the cursor is not in a Java
  file, then the command won't be available if it were defined in a
  ftplugin dir.

Testing:
Verified the debug commands, status command.