Track app usage with (anonymous) analytics

[tofumatt]

I figure we'll use GA, but there might be better app-specific libraries out there we could use if anyone knows any.

But list of things we need to track are:

• [ ] Play/pause (to see usage patterns, how often people resume/listen in small chunks versus leave it on in the background)
• [ ] Check for new podcasts manually (we should make force refresh a feature and if a lot of users do it we need to update automatically more often)
• [ ] number of podcasts subscribed to
• [ ] episodes listened to completion (per time period + total)
• [ ] users
• [ ] app launches

And probably some errors too.

The analytics will need to be able to be recorded for later offline transmission. Huh, that could be a library and project to itself... scope creep!