Report unrecognized arguments from BENCHMARK_MAIN() macro (#332)

* BENCHMARK_MAIN() now reports unrecognised command-line flags (see google/benchmark#320)

* add benchmark::ReportUnrecognizedArguments()

Update BENCHMARK_MAIN() to use ReportUnrecognizedArguments() instead of
having the reporting code directly in the macro.

See issue google/benchmark#320 for reference

* let's stick to american english -- fix type in ReportUnrecognizedArguments()

* make ReportUnrecognizedArguments() print to stderr

* make ReportUnrecognizedArguments() return true if any arguments have been reported (i.e. argc > 1)