Support 17X cellphone number in China (virtual ISPs)

China has recently allowed "virutal mobile ISPs" and assigned them with cellphone numbers starting with 17 (still 11 digits in total).

I've updated the regular expression, doc and tests.
Could you add a `versionchanged` statement about adding 7 in here? See https://github.com/django/django-localflavor/commit/dc148426a5bbcbc0f1690190a6f595ce3aae6bc6 for an example.

Also, while we're updating this, let's patch up the language a little. I suggest:
* the second could -> the second should
* and 8 -> or 8
* the rest could be what so ever -> followed by 9 more digits