Add support for parsing an Uri to OtpUri

[xZise]

This adds support for extracting the different values from an Uri for an Otp. It only accepts valid input combinations/values and failes whenever it doesn't look like an correct uri.

[xZise]

I implemented it for the constructor but maybe it should be a static method like TryParse() or Parse. I was also thinking about whether to not fail as hard with issues (e.g. when providing a counter to when using TOTP) or to disable the exceptions via a parameter.

This might also implement #66.