`Toggle symbol info at status bar` command

This is an alternative way of implementing a suggestion I gave at #134.

It adds a `Toggle symbol info at status bar` command that 
redirects `Show info` between the current panel and the status bar.
 
If a `sublime_plugin.EventListener` to call `sublime_haskell_symbol_info_command` periodically updates the status bar, this becomes a faster way to read types while browsing a file.
I plan on at least adding another commit where this happens by calling it after a delay where no move events fired.

Can anyone review this?
In particular, this down at autocomplete.py looks funky at the moment:
```python
show_status_message(decl.detailed().split('\n')[0])
```

(I understand this is not a TODO within the project, but maybe it's a valuable feature to you.
I miss this functionality from Emacs and didn't find a command within that did it)