support ruby 2.0

Hello,

Current grit does not work because 
 * version checking condition does not work for ruby-2.0.
 * PACK_IDX_SIGNATURE string is treated as UTF-8 in ruby 2.0.

I confirmed that tests still pass on ruby 1.9.3p392 and on ruby 2.0.0p0 with this change. FYI to run tests on ruby 2.0.0p0 we also need to modify mocha like https://github.com/freerange/mocha/commit/cf91bded89fe2172f2f82146aaacaa520f564b0 .

Kazuhiko