Parsing large floats on Windows is slow

[lamont-granquist]

See #50.

Nobody has reported hitting this issue, and for ffi-yajl's use in Chef we don't see how that would likely happen, but if anyone is trying to parse 8 million digit long floats with this library you'll likely have to wait a very long time. The perf bug is likely not in this library but probably in Ruby itself or in the Windows libraries that Ruby or the libyajl C library is using.