fixes Issue #284 - call GridDataSource.prepared() before calling GridDataSource.getRowValue()
fixes Issue #283 - DataTable's row parameter shouldn't be a required parameter
fixes Issue #285 - do not use the environment to give access to the current DataTable's row and rowIndex parameters
Merge pull request #286 from ascandroli/master

small DataTable refactoring (patches for #283, #284 and #285)