update to std:string usage

[danomatika]

The OF API has transitioned to explicitly using the std namespace when declaring types and this PR updates the ofxSyphon API to use std::string instead of string. This should have no effect on existing projects.

Also, I'm building ofxSyphon as a Lua module for an OF + Lua interpreter and the bindings generation works with std::string, so this help me out. :)

[danomatika]

Any interest in this? It's a harmless change and updates the API to better match OF's which uses std::string with namespace throughout.

[bangnoise]

Thanks!

[danomatika]

Cool, thanks.