Modify zh-hk to use formal Chinese writing instead of Cantonese

The present zh-hk translation, which is in Cantonese (spoken / colloquial in writing), is not suitable for use in most developments.  The written form of Chinese used in the formal context of Hong Kong is very similar to those in Taiwan and overseas.  In this PR, I have adopted (with some stylistic touch-up) the zh-tw translation into zh-hk, and moved the existing zh-hk to zh-yue, which refers to Cantonese.

For a more detailed discussion of the topic, see e.g.  https://drupal.org/node/1975400. A similar scheme (i.e. zh-yue for Cantonese, zh-hk for (Written) Chinese (Hong Kong)) is now used by Drupal.

Thank you.