Compression (#147)

* enable header detection in util::decompress and also use it for gziped data

* expose zlib windowBits so mbgl::util::compress can be used for different formats

* also expose windowBits in decompress function for consistency

include/mbgl/util/compression.hpp

@@ -5,8 +5,10 @@
 namespace mbgl {
 namespace util {
 
-std::string compress(const std::string& raw);
-std::string decompress(const std::string& raw);
+enum CompressionFormat { ZLIB = 15, GZIP = 15 + 16, DEFLATE = -15, DETECT = 15 + 32 };
+
+std::string compress(const std::string& raw, int windowBits = CompressionFormat::ZLIB);
+std::string decompress(const std::string& raw, int windowBits = CompressionFormat::DETECT);
 
 } // namespace util
 } // namespace mbgl