Fix parsing issue with quoted shorthand values

Fixes #174.  The problem was that if multiple equal
signs are present in a key/val pair (if the value has
an equal sign) then we would hit this error.

See [here](https://github.com/jamesls/aws-cli/compare/fix-174?expand=1#L2R50) for an example of this.

I double checked the other parsing parts of argprocess, they all
properly use '.split("=", 1)'.

This also required updated the csv parsing to handle the
case where you quote a value without commas, e.g. foo="bar=baz".

cc @garnaat @toastdriven