Ruby 1.9.3 support

I know that the README says this is a Ruby 2.0 project, but the changes necessary to run under 1.9 are pretty minor.

All specs pass for me against  1.9.3-p484 and 2.0.0-p353, though I haven't dug into how good the coverage is yet to see if there is anything missing.  Let me know what you think.