Unit testing for PTF

[shippy]

(copied over from #63)

Must-have unit tests:

• (Dollar) formatters get the right value
• Missing values in catch trials filled correctly
• Correct lookups
• Export subjects
• Export fails gracefully for multiple versions of DataObjects
• Export manages incomplete subjects
• Divide trials into blocks correctly
• getReferenceAndValue gets the right reference
• getBlocksForSession gets the right blocks