DISPLAY_IGNORE now works on a line-by-line basis as DISPLAY_LINES_IGNORE
Refactored nbtest so NBTester class defined *after* cells are executed

Defining NBTester class after cell data is collected as the execution of
cells can potentially clobber the namespace (and break the test
class). In particular, Topographica's load_snapshot command would break
the definition of NBTester causing nosetests to fail.