Added an openstring function, by wrapping the string in an object
that supports read(bytes) and passing the object to the underlying
code. Modified all the tests to run with both open and openstring.

Changed the default buffer size in csv.lua to match the value in the
README, and added the crucial blank line to test-data/blankline.csv
Merge pull request #1 from kev82/openstring

Addition of csv.openstring plus other minor fixes