Add gzip header check. Closes GH-6

Based on @kmpm's commit at kmpm/nbt-js@d4675211b28c489dcd9759bc229bd7fc1efa4c24#diff-f482261c886683a585ab8e53d2ef335bR143
Change to only decompress with gunzip (not deflate) for browserify compatibility
Merge pull request #7 from deathcap/headercheck

Add gzip header check. Closes GH-6