Zlib enhancements

Bug fixes and addition of an interface to deflateParams() for changing the compression level and strategy after intialization.
@mscdex Just as a follow-up to this conversation, we actually ended up upgrading zlib to 1.2.8 for the 0.10.34 release. More info here: http://blog.nodejs.org/2014/12/17/node-v0-10-34-stable/.