Adding Link widget
Adding directional link widget
Fix whitespace
Updated example notebook
Load link widget javascript
Load the link models
on("destroy",...) -> once("destroy",...) so we don't keep a reference to it, preventing gc

Thanks to Sylvain Corlay for the suggestion.
only check for formatters on instances

use inspect.isclass to disallow unbound methods,
not type checking on the method itself
Something
Make selection widget respect order.
Import Tuple traitlet for Selection widget
Fix variable name
remove _valid_formatter

check in _safe_get_formatter_method

- reject classes
- allow any callable...
- ...except on objects that lazily produce callables at any attribute
Fix readonly lock logic.
Don't magically convert values to a list of tuples.
Fix faulty interact tests
Typo fix
Allow FileLinks to not recursive into subdirectories
Remove unneeded modules, and added docstring info on recursive parameter
Selection test fix