use system Normalizer class if available, fall back to Patchwork if not

Patchwork's Normalizer class is billed as a pure PHP fallback implementation of the original function from php-intl, but OwnCloud always goes straight to using Patchwork's. This checks if the function is already available from php-intl and uses it if so, otherwise it falls back to the Patchwork implementation. This is good for downstream distributors who want to avoid bundling Patchwork.

(Note: I am just the QA monkey and really have no official clue what I'm doing. I have never learnt to code, I just cargo cult stuff around until it works. I threw a couple of debugs into this patch to test it and verified at least that if both php-intl is installed and Patchwork is available, it finds the php-intl function and uses it, and if php-intl is not available but Patchwork is, it correctly falls back to the Patchwork function. I have no idea if this is a really stupid way of doing things, please tell me if it is).