Add option to read from IO objects in addition to files

[zhum]

This is complicated task, because it needs to put Ruby IO methods inside C-code... But merge requests are welcome!

[jpmckinney]

I just wrote my own library https://github.com/opennorth/fastcsv as all the others either had errors, were incompatible with Ruby's CSV, or were slow. Benchmark: https://github.com/jpmckinney/csv-benchmark

And it reads either Strings or IO objects (e.g. a File).