Use camel case for element attribute and event names

[gnestor]

Broken out from #281

This will allow phosphor’s virtualdom to support rendering React functional/stateless components.

To do:

• Switch from no-case to camel-case for attribute and event names
• Name parity with React
  • React's list of attribute names: https://github.com/facebook/react/blob/b1768b5a48d1f82e4ef4150e0036c5f846d3758a/src/renderers/dom/shared/HTMLDOMPropertyConfig.js#L28-L207
  • Phosphor's list of attribute names: https://github.com/phosphorjs/phosphor/blob/master/packages/virtualdom/src/index.ts#L27-L124
  • React Typescript definition file from DefinitelyTyped: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts
  • fuelDOM (React-compatible) Typescript definition file: https://github.com/brn/fuel/blob/dc7a2025407686dc086abe90d0a423f223b76fa5/lib/type.d.ts

[ellisonbg]

Can we allow phosphor to accept the lowercase and react style names to avoid forcing a 2.0 release?

[sccolbert]

Yeah, we're gonna have to do something like that.