Csv presto

I recently needed to use CSV return type, but Presto didn't seem to support it. 

For one, its not a recognized MIME type, and the default case was returning content type as `application/csv`... this is incorrect according to RFC 4180:

> This RFC documents the format of comma separated values
   (CSV) files and formally registers the "text/csv" MIME type for CSV
   in accordance with RFC 2048 [1].
>> [Source](http://tools.ietf.org/html/rfc4180#page-2)

I also registered a default type handler (similar to `text/plain` it just prints the `$dom`)