refs #615: sendkeys does not work with contenteditable

This fixes an issue where sendKeys does not fill in the text on
contentEditable elements. The element is checked in the attributes
object whether `contenteditable` is true and clicks the element to
sendKeys to.
Document elements supported by sendKeys
Merge pull request #620 from mduvall/content-editable-sendkeys

refs #615: sendkeys does not work with contenteditable