Ensure the original order of dependencies is kept

[pombredanne]

This is not super important but helps: deps are tracked in mappings of scope->set of deps.
It would be useful to keep these in a list (eventually ensuring items are unique in the list) to keep as much as possible the original order. This help when comparing results from a parse with the original POM.

[wfscheper]

I'd accept a PR that used an OrderedDict instead of the builtin.

[pombredanne]

@wfscheper I will, as clean commit off the https://github.com/sassoftware/pymaven/tree/clientless-pom branch