port to Ruby1.9+ a little bit further

Hi!

To have ruby-fusefs work with ruby1.9.3/ruby2.0 on Debian, I had to do the following changes. Please consider adding this patch. It built fine and I managed to run a couple of examples (hello.rb, demo.rb, yamlfs.rb).

Could you please consider releasing a new gem, at least for 0.7.1? The latest available 0.7.0 still has the typo RSTRING_LEN instead of RSTRING_PTR in the code.

Thanks!