Fixture: add non const Setup() and TearDown(). (#285)

* Fixture: add non const Setup() and TearDown().

This allows write-access to the State variable, which is important in
upcoming user-defined counter functionality.

* Fix const placement in the Fixture methods.

* Fixture: use const_cast instead of static_cast.

include/benchmark/benchmark_api.h

@@ -709,8 +709,12 @@ class Fixture: public internal::Benchmark {
       this->TearDown(st);
     }
 
+    // These will be deprecated ...
     virtual void SetUp(const State&) {}
     virtual void TearDown(const State&) {}
+    // ... In favor of these.
+    virtual void SetUp(State& st) { SetUp(const_cast<const State&>(st)); }
+    virtual void TearDown(State& st) { TearDown(const_cast<const State&>(st)); }
 
 protected:
     virtual void BenchmarkCase(State&) = 0;