added native support for loading CSV files

This commit adds support for "native" CSV files backed by
'clojure.data.csv' in order to have RFC 4180 compliant reading.

This especially helps with CSV files that mix or have quoting for the
cells which cannot easily be solved by using the TSV supported
delimiter.
added quotes in cell testcase and clarified documentation
Merge pull request #100 from sarnowski/csv-support

add native support for loading CSV files