Minor tweak: Test memory as strictly as possible.
more memory testing'
ENH: link_generator can step through linking one level at a time.

Used with a special subclass of Track, this allows unlimited levels
to be linked using a limited amount of memory.
REF: Major refactor, basing link on link_iter and adding link_df.
Fix misspelled examples diretory.
BLD: Add pandas dependency to Travis.