Today Extension for simple stats view

Closes #2278

**This is a new pull request** (old request: #2314)

This implements a "simple" today extension to show a single site's stats in the today/notification center area. Some of the challenges met:

Today Widget does a small network call to keep up to date separate from the main app. To support this we have to store the following items in a central/shared area: ** WordPress.com OAuth2 Token ** Site ID ** Site timezone (for fetching "today")
Provisioning profiles have been a complete nightmare with this. The widget target needs its own app ID in Apple's provisioning portal (I think). Seems like keychain sharing is finally working right. This may be brittle in the future.
The state of the widget is a bit funky once in a while - most often only upon first launch of it. We may need to get some eyes at the constraints in the storyboard. Widgets have no defined height so it's interpreted through the constraints themselves which means it a little different than usual constraints.
Tapping the text in the widget launches the app. I can't get the whole thing to be tappable - this may be a beta 5 bug. May need to turn the whole thing into a single-row UITableViewController if the GM doesn't fix this.
Haven't thoroughly tested this on iPad.
The interaction to get the site stats in the Today Extension -> go to your site stats in the main app and tap the "Today" button.

@sendhil
Instead of a comment describing this if statement, perhaps refactor to use some `BOOL` values? i.e.

```
BOOL notPresentedModally = self.presentingViewController == nil;
BOOL widgetsExist = NSClassFromString(@"NCWidgetController") != nil;
if (notPresentedModally &&widgetsExist) {
```