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. 
Both of the formats are ISO 8601 formats for durations, so this language is unclear, though the reference to the particular section of ISO 8601 helps clear it up. Proposed altered language: "The Duration property SHOULD* be expressed using the format for duration in ISO 8601:2004(E) section 4.4.3.2. The alternative format in 4.4.3.3 SHOULD* NOT be used"