benchmark

Add Setup/Teardown option on Benchmark. (#1269)

* Add Setup/Teardown option on Benchmark.

Motivations:
- feature parity with our internal library. (which has ~718 callers)
- more flexible than cordinating setup/teardown inside the benchmark routine.

* change Setup/Teardown callback type to raw function pointers

* add test file to cmake file

* move b.Teardown() up

* add const to param of Setup/Teardown callbacks

* fix  comment and add doc to user_guide

* fix typo

* fix doc, fix test and add bindings to python/benchmark.cc

* fix binding again

* remove explicit C cast - that was wrong

* change policy to reference_internal

* try removing the bindinds ...

* clean up

* add more tests with repetitions and fixtures

* more comments

* init setup/teardown callbacks to NULL

* s/nullptr/NULL

* removed unused var

* change assertion on fixture_interaction::fixture_setup

* move NULL init to .cc file