Update export-csv.js

Add text for 'Download CSV' from highchart language options if present else default back to default. This will help in websites supporting multiple language

![screenshot from 2013-12-03 13 15 35](https://f.cloud.github.com/assets/2534747/1661265/fef7549a-5bd9-11e3-9c09-240bd7118cd6.png)