Make DateFormatter work in any language, by default in the page content language

Make DateFormatter work in any language, by default in the page content language

Add a lang parameter to DateFormatter so it can work in any language instead of only the site content language.
(The memcached key is now per language code.)
Use by default parser->getTargetLanguage() so it is parsed in the page content language by default.
Also add some documentation and remove unneeded whitespace.

If needed, a parameter to {{#dateformatter}} can now be easily added, to specify the language to format in.

Change-Id: If61854920065f7c3b4170ab89e9aa66b299f9dd8