Improve buffer-alloc benchmark performance

[olegp]

http://olegp.github.com/common-node/graphs/buffer-alloc.png

Results are far worse than other benchmarks, most likely due to the slow zero() function. Should try using clear from https://github.com/bnoordhuis/node-buffertools/ or drop the requirement to clear the buffer (this would break tests though).

[olegp]

clear() from buffertools was also very slow, so have decided to stick to current zero function

will add an extension to bytearray constructor which will let us create buffers without clearing for performance critical code

[olegp]

This is not due to clear(), since results without clearing the buffer are the same.

Current benchmark (with clear()) is at http://olegp.github.com/common-node/graphs/buffer-alloc.png

[olegp]

This was due to use of Object.defineProperty, fixed.