Throw a new Error, not string

See http://www.devthought.com/2011/12/22/a-string-is-not-an-error/
Use built-in zlib module instead of compress. Closes GH-2
Add int array (type 11) support. Closes GH-3
Merge pull request #4 from deathcap/zlib_ints

Add TAG_Int_Array support and update to use zlib