Merge pull request #2 from danielballan/master

Transfer notebooks from trackpy and include sample data as PNGs.
Fixed typos in README and walkthrough
Added performance notebook
Add link to browse all notebooks, and info about contributing
Merge pull request #4 from nkeim/revise-readme

Revise README
Import matplotlib in performance.ipynb
Merge pull request #6 from nkeim/fix-perf-nb-imports

Add "%pylab inline" to performance.ipynb, in case pylab not imported on startup
Update and expand section on PIMS; add review of advanced features.
DOC: Update walkthrough
Merge pull request #7 from danielballan/update-walkthrough

Update walkthrough
add foam
Merge pull request #12 from sciunto/foam

bubble velocity field in foams.