Allow for duplicate properties

[omus]

Overview of the work done here:

• Allow for duplicate properties with the same name. There's nothing in the JUnit XML schema that requires unique property names and I've seen some examples where duplicates are used. This change is not backwards compatible with the previous behavior which allowed nested properties to override their parent set values.
• Updated the flattening of testset properties such that parent properties appear first. Consistent with Flattened testsets ordered by appearance #103.

Extracted from #106 to make reviewing easier.

[oxinabox]

This one is definately breaking. Which is fine.
I agree with the spirit of this

[oxinabox]

LGTM
merge this and rebase the other and i will wrap up review of that then we can tag a breaking release