Update version number
Add ability to combine Commands

Combinable commands must return true for a canCombine() method and must
implement a combine() method that combines one command with another
command of the same type.
Update canCombine() to have a parameter
CommandHistory now attemps to combine by default
SetAttributeCommand is combinable
Delete history if combining non-final command
ChangeTag/TextCommand require same element
ChangeTagCommand now combines
ChangeTextCommand combined command is new text only
Un-deprecate ElementEvent#getElement()
Undo no longer causes exception from observation
Adding an attribute no longer causes a stack overflow
Typing an attribute value no longer un-focuses the field