Add support for event constructors

Currently these are implemented:

new Event
new CustomEvent
new UIEvent
new MouseEvent
new FocusEvent

Adding new ones should be easy but we should have the generated event wrappers
generate these.

One issue here is that IE10 does not support event constructors. I'll file a bug
about polyfilling those as needed.
Merge pull request #57 from arv/event-constructors

Event constructors