Visualizations refactor

I've separated each visualization declaration into its own module, so each visualization defines how to render it and what options it needs. The base visualization render/edit directives are not aware of the visualization types that are available.

Adding a new visualization requires:
1. Write the module.
2. Add the module to the `redash.app` module dependencies.
3. Include the file in the JS files list in `index.html`.

I would love if it could be a single step, but not sure how to make #2 and #3 automatic (specially #3).

Would love to get comments and ideas.