buffer: floating point read/write improvements

Improvements:
* floating point operations are approx 4x's faster
* Now write quiet NaN's
* all read/write on floating point now done in C, so no more need for
  lib/buffer_ieee754.js
* float values have more accurate min/max value checks
buffer: improve read/write speed with assert (unstable)

Assert checks can be performed faster in C than in js. So a set of
generic methods have been created on SlowBuffer that will be called if
the user doesn't pass noAssert.
Merge commit 'd3115cb' into buffer-noassert-c

Conflicts:
	lib/buffer.js
	src/node_buffer.cc