TODO
- test both ffi and ext on platforms that support both (MRI, rbx)
BUILD NOTES
- building libyajl2 requires 'apt-get install cmake' on ubuntu, but note that 'apt-get install libyajl2' will be better
KNOWN BUGS
- 'rake compile' broken on mac, only tested to work on linux (ubuntu)
- C Extension segfaults on ruby 1.8.7, so the ffi mode is forced for RUBY_VERSION < 1.9