Move xmldom parsing to internal interface

[pgoldrbx]

Fixes #18

Changes

• Create an internal interface to wrap the xmldom parser.
• Add unit tests for new interface (maintain all existing tests, despite some redundancy)
• Update helpers tests to leverage this tested interface

Outstanding questions

• Using this interface in the helpers tests simplifies things but adds some interdependency. This is arguably not a great decision. It's worth some debate.
• The parse method could have a better name? parseXML or parseFromString (like the upstream library)
• Should the interface be more thoroughly tested so we don't need to know/care about the upstream dependency? or is it ok to simply stub the xmldom methods and rely on expected behavior and output?

[pgoldrbx]

I notice now that we're not logging the exception raised by the parser. However, I think this is better addressed in #14

[taveras]

@pgoldrbx this LGTM!
i rebased this branch, fixed some lint errors, and updated the changelog.