Add extern "C" to headers when being compiled in c++.

[Icantjuddle]

I've been using fzy as a library to do some fuzzy finding in a program I've been working on, this (and a few other small tweaks I'd like to make with your permission) would make it much easier to use in a larger project.

If you're ok with this I'd also like to add:

1. non-global namespacing when being used in c++ (i.e. add open/close namespace tags)
2. Some documentation to the headers.

[jhawthorn]

Thanks!