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
remove _valid_formatter

check in _safe_get_formatter_method

- reject classes
- allow any callable...
- ...except on objects that lazily produce callables at any attribute
Allow FileLinks to not recursive into subdirectories
Remove unneeded modules, and added docstring info on recursive parameter
Kill the layout manager
Fix scrolling for large outputs
Fix scroll/resize handle missing behavior.
Fix, prevent notebook from having its height set on pager display
Increase pager handle size.
remove special handling of ipython_kernel in launcher

- set JPY_PARENT_PID and JPY_INTERRUPT_EVENT env variables for all kernels
- no custom kernel args other that console passthrough
- use devnull instead of `--no-stdout` (comments claim it shouldn't work, but tests suggest it does - let's keep an eye on this)
moving, checking MathJax typesetting to WidgetView

widget rendering should not fail hard if MathJax is not available
also setting text in widget.typset
reversing order of arguments, as text may already exist