Implement to unmarshal PT duration

[miyukki]

Refer to #23

go-dash output the MPD with PT duration, but could not parse its notation.
This PR resolve an above issue.
However, It might be including the mistake due to I can not read the full specification of MPEG-DASH.

[philcluff]

Thanks for the PR! I'll take a look today.

[thomshutt]

Hey @miyukki - sorry it's taken us a while to look at this!

Overall it looks good, but I noticed a few things:

• Could we have some error test cases too please? There are some good examples at http://www.datypic.com/sc/xsd/t-xsd_duration.html

• The above link also has valid cases for when this starts with things other than PT, which will currently be rejected by your parsing code

• We also need test cases for when the field is an integer/float.

• I think we probably want to guard against negative durations.

[miyukki]

Thanks your review, and I saw the good examples about xsd:duration.

I changed the parsing code, but I got an issue about duration.
xsd:duration can be represent the duration with Years and Months, but it couldn't convert to golang time.Duration because Years and Months are not fixed length of days.
So I calculate the duration with beginning of unix time.
Please let me know if you have any specifications about it.

[thomshutt]

@miyukki Good point - I can't find anything in the spec about this, so I guess we should just reject durations that contain Years/Months

[miyukki]

@thomshutt Sorry, it took so long. I fixed PR. Please review.

[thomshutt]

Thanks @miyukki, looks great!