* 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
