Track event callbacks

[yuhao]

We can track addEventListener, but do we intercept whenever a callback is triggered? Need to think about various ways that a callback can be registered.

[yuhao]

This patch allows us to intercept callbacks that are registered through addEventListener. This misses events that are added through writing onevent properties.

This article discusses ways that events could be added. Note that events that are added through jQuery eventually call addEventListener.

[yuhao]

If a callback is registered through el.onclick = foo, it should already be captured -- we just have to add instrumentation APIs around it.

If an callback is registered inlined within HTML such as <p onclick="foo()">, then we won't be able to capture it.