Added support for adding a data point at a specified index.

git-svn-id: http://achartengine.googlecode.com/svn/trunk@509 0b5bfab6-fc16-11dd-ac65-f74f249cfaac
git-svn-id: http://achartengine.googlecode.com/svn/trunk@510 0b5bfab6-fc16-11dd-ac65-f74f249cfaac
Merge branch 'master' into HEAD