Nicer annotation format in default exported csv

[cbogart]

When an annotation has only a single, null-named feature, don't display "null" in the field; display annotation name = null feature value.

configuration	example
Single null-named feature:	transactivity=3
No feature	transactivity
Named features	transactivity (measure1=3, measure2=7)
Both named and null feature	transactivity=5 (measure1=3, measure2=7)