Widgets 

This adds `HandsonTable`, `PivotTable` and `DatePicker` (also `Tabs`, but that's just a stub right now) widgets, `Date` property, `app/downloads`, `glyphs/widget_server.py` and `glyphs/data_tables_server.py` examples. There are improvements to `ObjectExplorer`. ~~This PR merges in `sbt` branch (just for my convenience)~~. This PR is ~~very~~, very, very (or even more) WIP. No point in reviewing right now, but you try running the examples (`app/downloads` requires data; I will send an E-mail tomorrow how to obtain it). 

There are lots of issues with this PR. `HandsonTable` has known issues with rendering, so you may need to resize the window to make it render better, but at least it shows up, which wasn't the case at the beggining. ~~I had to `from .widgetobjects import *` in `bokeh.objects` to help `Viewable` discover classes (first import counts and without this hack you will get missing models errors)~~. I had to include, hopefully temporarily, bootstrap.css to make `PivotTable` render (almost) properly. I tried to use `reference` less imports, but without success. `glyphs/widget_server.py` allows data editing through `HandsonTable` and line glyph is nicely updated, but not `DataRange1d`, which doesn't follow data modifications.