Allow Chinese characters in Topic titles

Thanks @kejinlu. Fixes: [meta/6883](http://meta.discourse.org/t/i-found-that-i-can-t-post-when-the-title-contains-only-chinese-characters/6883/4)

Chinese titles are currently not allowed because of the way we validate word length in titles. There are no tests indicating that we want to count dashed-words or underscored_words in titles as separate words, so this is an easy fix that validates word length in titles based on spaces instead of non-word characters.