py.bench -- py.test based benchmarking

py.bench -- py.test based benchmarking

The idea is to reuse py.test infrastructure to

  o collect tests
  o run collected tests
  o present test runs results

and substitute 'test' -> 'benchmark'.

What we do is

  1. collect benchmark files that start with 'bench_' prefix
  2. execute two kind of benchmarks

     a. 'bench_<smth>', and
     b. 'timeit_<smth>',

for bennch_XXX we simple mesure time to execute, and for timeit_XXX in
essence we do IPython's %timeit on it.

For actually running benchmarks small py.test like utility is provided
and installed for the benefit of all.

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: Ondrej Certik <ondrej@certik.cz>