Merge entity finding code into single classes

[schmich]

We should have a single class for finding entities, i.e.:

• Move Stream.get into Streams
• Move Channel.get into Channels
• ...

Eventually, we'd have something like:

• Streams.get('lethalfrag')
• Streams.find(...)
• Streams.all

Update docs, update specs.

[schmich]

Fixed with API update.