Clarify wording/naming of blank rows export option - fixes issue #651
- clarify that it refers to all non-null cells
- rename variables without compatibility constraints to match actual
function
Ignore test-output directory
Update OpenCSV to our fork with multi-character separator support

Built from the openrefine-2.6 branch in the OpenRefine/opencsv repo
Switch escape character from NUL to DEL in hopes that it's rarer.
Add some tests before working on #640 and #409
TODO about memory usage
120 pixel icon for Google OAuth2 registration