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ARROW-1333: [Plasma] Example code for using Plasma to sort a DataFrame

This provides the example code used to generate the speedup mentioned in #940. It is also an end-to-end example using the Plasma object store.

Author: Robert Nishihara <robertnishihara@gmail.com>
Author: Philipp Moritz <pcmoritz@gmail.com>

Closes #942 from robertnishihara/sortdf and squashes the following commits:

a94ce907 [Philipp Moritz] add licenses
221b3362 [Robert Nishihara] Small cleanup.
0cba9904 [Robert Nishihara] Small naming changes.
94e85ee8 [Philipp Moritz] move files
07a53b36 [Philipp Moritz] add experimental setup
b39caacc [Philipp Moritz] remove original multimerge function and document multimerge2d
4df2accf [Philipp Moritz] batch calls to object store
0f7c44c6 [Philipp Moritz] small cleanups
8d997786 [Robert Nishihara] Fix indentation in cython.
f516ab7b [Robert Nishihara] Small fixes.
1f74047d [Robert Nishihara] Speed up Cython
6ef8a776 [Robert Nishihara] remove some printing
e830587a [Robert Nishihara] Fix bug.
a92996a2 [Robert Nishihara] Update plasma executable path.
fc6707f4 [Robert Nishihara] Fix sorting example (fortran versus C arrays).
5d3c74aa [Robert Nishihara] Almost working sorting dataframe.