Add speaker and tag API

[beenje]

With the current API, you can request videos for a specific speaker or tag, but there is no way to know the existing speakers or tags.

This allows to retrieve the list of speakers and list of tags.

[willkg]

List of speakers is definitely helpful. The tag data sucks, though. I don't want to expose it because it's a bunch of terrible data.

There have been some ideas on how to do a better tagging system, but I think work on that is going to be a ways out.

What's the use case for looking at the tags data?

[beenje]

I'd like to fix the XBMC add-on for pyvideo.org. I thought about giving a list of videos per category/speaker/tag. If the tag data is not well organized, it probably won't be useful. So no problem to drop that.

A real search by query like on the website would be much more useful. I will try to look a it.

For this PR, would it be ok if I remove the list of tag and keep only the speakers?

[willkg]

Keeping the speaker stuff and removing just the tag bits would be perfect!

[beenje]

Tag bits removed

[willkg]

I squashed them and landed it in master: 760bcac Add speaker API

Thank you!