Fix int64_t_t typo in README code example

Fixes a typo where the first type was written as int64_t_t with suffixing _t twice and the second type misses suffix at all.

Ensure we run at least 1 iteration.
Take into account overhead of an empty benchmark.
Minor cleanup of code.

Benchmark library builds and runs but only single-threaded. Multithreaded
support needs a bit more love.

Currently requires some C++11 support (g++ 4.6.3 seems to work).