More sensible handling for page existence
* Page.exists is now None if we haven't actually checked
* Functions that require the page to exist now actually verify that
  it does
* Remove a lot of useless code that checked for the impossible
  condition of no title and no pageid
Rename self.wiki to self.site in APIRequest for consistency with
everything else.
Don't use 'type' as a variable name
Fix queryGen - changes for switch to requests
Migrate category to Python3, plus a general overhaul
* Add getCategoryInfo() to get the number of pages in the category
* Don't store the category members, as that defeats the purpose of
  having a generator function
* Use queryGen() to get the data