Wrap getComputedStyle

Fixes #64
Implement event constructors for Safari and IE

This implements event constructors for

Event
CustomEvent
UIEvent
MouseEvent
FocusEvent

More can easily be added as needed.

Fixes #58
Update README.md
Override document.{body,head}.{appendChild,insertBefore,replaceChild,removeChild}

To be able to support this the Node methods can no longer call
`this.impl.appendChild` and instead we call `originalAppendChild.call`.
Merge pull request #89 from arv/html-body-head-element

Html body head element