Legend text dimension cache is not cleared #775

Hello,

Legend item text rect dimensions cached in legendItemTextBox are not cleared if data names are updated. This results in incorrect legend position after calling chart.data.names().
http://jsfiddle.net/7fu8tydq/1/.

Problem was introduced in commit 1c35022 in version 0.4.4. There are probably similar problems for other performance fixes from issue #730.