Added simple example and performance comparison.

* Added very simple example program that demonstrates how to use Arpaca.
* Added program to compare the performance of Arpack and Eigen.
* Both programs come with compile instructions and do not rely on a
  build system or other dependencies.
Preliminary IPython notebook on performance.

* Preliminary IPython notebook with analysis of the performance of
  ARPACK and Eigen.
* Minor improvement to command line argument handling in
  performance_plot.