Bootstrapping widgets from javascript initializer.

[akuhn]

Bootstrapping widgets from a javascript initializer. Introduced a px object with registerWidget function to register a widget initializer and initializeWidget which is called on document ready and returns an object with {render, resize} functions.

Migrated word cloud, big number and nvd3 widgets.

Follow-up work

• Migration of table and markup widget, they serve static html and will need extra migration work.
• Break visualization into separate render and resize function, where resize doesn't fetch data.

Test plan

• Load example dashboard.
• Load all example slices.

Reviewers

• @mistercrunch

[mistercrunch]

This looks very reasonable, let's babysit the app after we merge it

[mistercrunch]