Implement gzip compression in the C++ layer

[danpat]

This change goes towards #413, adding

tile.getData({compression: 'gzip', level:9, strategy:"DEFAULT"});

options so that vtile compression can be done in the C++ layer, rather than passing full data out to NodeJS.

Points to review:

• not clear on all the return types, do they need to be wrapped in NanEscapeScope()?
• private method _gzip_compress, is there a better place to put this?
• data types passed to and returned from _gzip_compress, are they appropriate in C++-world?

[springmeyer]

going to close this up now. I've manually integrated this work into #505 - thanks @danpat!