Feature/access custom namespaces

[hotdang-ca]

I've noticed a bit of a trend -- new namespaces are introduced continually. And, this can continue.

But for now, I need support for:

• namespaces not yet introduced (I'm parsing out g-core items... no idea what that is)
• namespaces used incorrectly (my example code declares itself as an Atom feed, but does not follow the spec. I had to load it in as an RSS feed instead

This request adds support for reading in the raw text of unidentified tags (well... ALL tags at the moment. I'd love feedback regarding what to exclude).

This allows me to access:

...
<item>
  <g-core:price>2499.99</g-core:price>
</item>

via

feed.items!.first.customNamespace!.valueForKey('g-core:price')