Make 'flush' implementation optional on the output stream object.
Ensure that Node.js buffers are zero-filled when created.

This helps prevent odd node-only bugs, since the Uint8Array is zero-filled
by spec.
Add compression-level options to lzmajs binary.
Tweak description in package.json.
Update mocha dependency for node 0.10 compatibility.
Release 0.9.1.