More changes in nomenclature

I'm working to implement the changes agreed this week, namely:

1. Changing URISet to iriset
2. Changing all element names to all lower case (includeHosts to 
includehosts) etc.

This is requiring more than a search and replace job but it's getting 
really tricky where the descriptors are concerned since the capital 
letter no longer distinguishes the block of descriptors from individual 
descriptors within it.

So, as much as an editorial plea as anything, I'd like to seek consensus 
on changing the element name from Descriptors to descriptorset. This is 
consistent with iriset and tagset.

OK with everyone?

P