shift tab for tooltip

As discussed on list,  use shift tab to trigger tooltip

This does *not* change the curent behavior, only add the shift+tab shortcut. 

Note that the shift tab shortcut has a slightly different behavior.  You can **select  part of a line** and pressing shift-tab will show you the **tooltip only for the selection**.

This is disabled for multiline selection to still allow to unindent block of code, Keep in mind that the real real shortcut for indent unindent is Ctrl+] or [ . Select/tab is not really supported by codemirror.

Finally the "tooltip_on_tab" behavior is globally configurable via IPython.config so that it could be easily switched to false. 

I can be overridden via js console for test purpose.
```
IPython.config.tooltip_on_tab = true | false
```
Take effect immediately, only on current notebook.


or via custom.js
```
var user_conf = {tooltip_on_tab:false | true};
$.extend(IPython.config, user_conf)
```