Improve handling of str values for Duration

[backbord]

Hi @mvantellingen,

I really like your project but recently stumbled into #876 when I wanted to specify a duration as "PT15M" and got "P%P" in the generated SOAP message.

Here's my attempt to improve the behaviour. Two simple and small test cases are included.
I hope you might consider merging the PR or giving advice if there's another desired way to do things.

In this PR, two things happen:

• Passing in a datetime.timedelta value is allowed.
• Passing in a str value will result in the str being parsed by isodate.parse_duration() to ensure the provided value is indeed an iso duration.
  The result of the parsing is then handed over to isodate.duration_isoformat() to produce the desired duration string.
  This round-trip mimics one of your test cases and results in builtins.Duration().xmlvalue("P0Y1347M0D") == "P1347M" where the superfluous zeros are eliminated. This should also ensure that the string is a valid duration in isoformat and raise a ValueError if it cannot be parsed as such.

Thanks and best regards,
Tim