Zlib enhancements

Bug fixes and addition of an interface to deflateParams() for changing the compression level and strategy after intialization.
@mscdex Because chromium uses zlib 1.2.5 and node's zlib bindings use chromium's zlib. Are there any critical changes between zlib 1.2.5 and zlib 1.2.8 that you think should be integrated into node's zlib?