Clarify ISO 8601 duration

Fixes #477. 

Note that the consensus between @fugu13 and @nashyura in the issue was that this requirement should be a MUST. I disagree for the following reasons:

* Russell mentioned that Wax supports the alternate format and has users using it. This is going to be a breaking change for those users. 
* The alternative format is allowed under ISO 8601 so long as there is agreement between the communicating parties. On that basis, I don't see how an LRS can be non-conformant for receiving alternate format durations. It could be flagged as non-conformant for returning alternative format durations as presumably the test suite does not have prior agreement to accept that format. Equally, a test suite for content could brand content non-conformant for sending using that format. 

I've used SHOULD* instead. 
Instead of "normally used" how about "in conformity with the format used for time points" (the language from 8601)?