Added new IT for callback

Comments has been added too
Added support for sDom parameter

Closes dandelion/issues#106
Null-safe securities added in XlsExport
Added support for sSelector parameter of the ColumnFilter plugin

Fixes dandelion/issues#107
HtmlHyperLink can now have an onclick attribute
Extended support for export using an AJAX source

New exportTag url attribute added, allowing users to specify a custom
export url
Linked with dandelion/issues#80