Double-quotes are escaped by preceding them with another double-quote

As per [RFC 4180 Section
2.7](http://tools.ietf.org/html/rfc4180#section-2):
> If double-quotes are used to enclose fields, then a double-quote
appearing inside a field must be escaped by preceding it with another
double quote.
Merge pull request #17 from lorcan/escapequote

Double-quotes are escaped by preceding them with another double-quote