[feature/faster]: Replaced Double.toString() with custom serializer function

While profiling my project, pbf2geojson convertor I have realized that around 50% of the time is spent with method dtoa() (See http://cr.openjdk.java.net/~bpb/7192954_4396272_7039391/src/share/classes/sun/misc/FloatingDecimal.java-.html), which is used in Double.toString()
I have changed the LngLatAltSerializer to write doubles without invoking this Double.toString()

Thank you for your work!