Add documentation to all event classes
Fix comment on MouseEvent.deltaX and deltaY: scrolling instead of delta movement
Add comments for TouchEvent and Multitouch
Better comply to the "Guidelines for Dart Doc Comments"

See https://www.dartlang.org/articles/doc-comment-guidelines/

* First paragraph should be a "one-sentence" description.
* Class descriptions are **nound phrases**.
* Function/method descriptions begin with a **third-person verb**.
Merge pull request #150 from marcojakob/master

Add documentation to all event classes