Add Benchmarks

[SteveCookTU]

Creating this issue for discussion started in #20

I was looking at the original library and noticed there are android benchmark images but it looks like the implementation was removed. I think we can turn some of the integration tests into benchmarks with some slight changes.

[axxel]

To whom it may concern: there is the little benchmarking project zxing-bench that I wrote to be able to compare zxing-cpp with other rust libraries for barcode scanning. It allows for simply throwing a file or directory (tree) at it and see how many symbols are detected in what time.