Tabbed interface and search bar

I've switched the layout here from the previous buttons, to the more familiar Android tabs:

![device-2014-01-17-151040](https://f.cloud.github.com/assets/231551/1941394/a761264c-7f89-11e3-9149-081f57ccfb7b.png)

Moving to a tabbed design also made me consider and subsequently move the search box to an action in the action bar - so tap the search icon, and the top bar becomes a search box.  This is consistent with apps like Gmail etc:

![device-2014-01-17-151426](https://f.cloud.github.com/assets/231551/1941429/41918a54-7f8a-11e3-81af-89e3cab65dce.png)

There's also a horizontal progress bar that shows above the tabs (but below the action bar) when display of events is taking place (but not necessarily downloading).

**Caveats**
The "Past" events still downloads all previous events, which as a result does tend to slow the app up a bit as it re-renders every 20.  We should definitely switch to some kind of incremental loading here eg load more events the further down you scroll, rather than loading in all previous 700-odd in one go.