Feature (issue #75): better ease of use / code cleanliness for supporting the EU service region

[Deconstrained]

This adds a new base_url keyword argument to every client class' constructor, which in turn sets the url property. All of the same validation rules apply and the old method of selecting a different service region still apply.

This also:

• (TODO) updates the documentation to more prominently feature service region selection.
• Adds support for new endpoints in the Jira Cloud Integration API that were added since the client was created

    "/accounts_mappings/{id}/rules",
    "/accounts_mappings/{id}/rules/{rule_id}"

See issue #75