TST: Another memory test. (No bugs, just being careful.)
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.
Merge pull request #13 from danielballan/link-variations

Merge Dan's link into trackpy, and refactor.