centering Legend in PieChart which has a big hole

1. fix getMaximumEntryLength in Legend
   I think the legend length should be ```formSize + textWidth +
   formToTextSpace``` and I don't really understand the former code.
   If I was doing something awful or messing it up please let me know.
2. I think the PieChart's legend should inform people what every single
   slice is. It is different from other charts like line or bar which
   shows several DataSet.
3. There is a big hole inside PieChart, maybe we can make good use of
   it. The sample code is updated and you can find out if this could
   be acceptable. Thanks.