add FastGetCountry func

[ahfuzhang]

FastGetCountry: when we need country only, this func is 2.91 times faster than Lookup

[oschwald]

This is a general purpose MMDB reader. I have no plans to add special methods for any particular database type. I think something like #91 or the existing deserializer interface should allow for manual control over the decoding when necessary. I also would be open to some sort of path lookup method similar to what libmaxminddb provides.