Stricter validation of constructor parameters

[carbonphyber]

I find libraries that raising Exceptions early and often when users pass in invalid/unsupported much more pleasant to work with.

I'm developing a partial rewrite of the jurisdiction.py constructor to raise exceptions if:

• the url parameter is invalid
• the string is not a URL
• the url string is not from a supported hostname

Since raising Exceptions in a potential future version where none are raised now is a breaking change, I'm submitting this Issue to gather feedback before submitting the PR.

[carbonphyber]

Fixed in #40