Make Feed keep and expose a reference to the original feed object

[infogulch]

Call Feed.OriginalFeed() to get the original feed if available.

Fixes #229

I just threw this together real quick to see what you think about it.

[infogulch]

Ah of course the test expected values don't match anymore. Not sure if the expected values should be updated or if the tests should ignore the originalFeed field since it's not exposed anyway.

[infogulch]

A different implementation could change the definition of Translate() to return three values: the general Feed and the specific feed object along with the error. Then there would be no need to add a field that affects the test result output. Though this strategy is not very backwards compatible to say the least.