Add more support to chart event code
- cleaned up hiearchy 
- now support 2 convoluted ways of event handling 
  a) directly cast the amchart event to ClickSliceEvent and invoke a
callback taken up via addListener code
  b) create/fire a ClickSliceEvent and use GWT event & handler mechanism

Signed-off-by: Sachin Shekhar R <sachin.r.r@gmail.com>