Add method for creating new tags

Add a method for creating new tags.  Since the response gives it for
free, also update the User object's tag list.
Require named arguments when creating tasks

When creating a task, use PEP 3102 to require everything after the
User instance be specified as a keyword argument, to avoid the
unintuitive `requests` argument getting in the way of the arguments like
`title` that you will almost always want.
Handle nameless tags

If a tag doesn't have a name, just store `Tag.name` as `None` rather
than hitting a KeyError.
Add a `Task.delete()` method
Add a `Tag.delete()` method

Since the API response gives it for free, refresh the User instance's
tag list, too.