Abstract string functions to support both mbstring and native

[wyrfel]

No description provided.

[coveralls]

Coverage decreased (-4.2%) to 95.756% when pulling 14c9327 on feature/string-functions into 9a54a71 on master.

[ejegg]

I'd really like to see this fixed - we've started using the library at the Wikimedia Foundation for parsing donor names, and Irish donors who write their name 'Someone Ó Something' are coming out with broken unicode.
wyrfel do you want to pick this back up? If you don't have time right now, I'd be happy to make my own fork and add the required test coverage on top of this change.