Cleaned up things in test_node.
Made simulator run for actual length of time given.

- before, it ran for one time-step less
Fixing test_node, since passthrough has no delay now.
Stopped divide-by-zero warning from occuring in test_lif.
Fixed bugs in test_lif and test_lif_rate (fixes #136).
Moved dt inside LIFRate.math(...).
Removed dead code