Enable to switch off the removal of comments in csv2rec.

This patch adds a possibility to pass comments=None to csv2rec in order to switch off comments removal.

A real world example are CSV files where the header on the first line is introduced by a hash #. As a hash is the default value for the comments option, the header is ignored. To read such files including the header one has to call csv2rec with something like comments='AStringThatNeverHappensToBeAtTheBeginingOfTheLine' which is not clear from the documentation and which seems more like a hack than a solution.

Currently, the docstring gives no hint about this behaviour and is misleading in the sense that it speaks about a character not a string:

    - *comments*: the character used to indicate the start of a comment  in the file