Remove simulated pull to refresh from most views

As discussed in #355, the simulated pull to refresh is distracting. This pull removes all of them except those in web views, which are handled differently.

This change led to a simplification of the API for syncing items in tables. I decoupled regular background syncing and user-initiated syncing, with the former now called via a simple `syncItems`. Manual syncing (e.g. via a pull to refresh) allows subclasses to do extra stuff if they need to.

I intended to preserve all "extra stuff" that's done by a manual refresh (including post pruning and the syncing of extra metadata), but a careful review is in order since this affects so many views.

Before/after:
![nopullrefresh](https://f.cloud.github.com/assets/515035/1498702/67f21d84-4840-11e3-86df-725dcd3a0753.gif)