Improve the performance of the CSV export plugin.

The previous version of the plugin scanned the entire csvData set
when replacing the last comma with a newline. This is a huge
performance hit for large datasets.

This version only scans the current row to replace the comma with
a newline.
Merge branch 'performance_fix_for_csv_plugin' of https://github.com/einarj/ng-grid into einarj-performance_fix_for_csv_plugin

Conflicts:
	plugins/ng-grid-csv-export.js
Merge branch 'einarj-performance_fix_for_csv_plugin'