Update email and defaultBlog for default WordPress.com account

Adds a new field `email` to WPAccount model. We'll send an update request to `/me` endpoint during login, just after defaultWordPressCom account is created or updated and it will update the email and defaultBlog information according to it.

I'll also add an update request to the new `MeViewController`'s `viewDidLoad` in [masterbar changes branch](https://github.com/wordpress-mobile/WordPress-iOS/tree/issues/2755-tabbar-enhancement).

Closes #2972.

@koke & @astralbodies: Can I bother you both with a very quick review? I was not sure about a few small things.

@astralbodies: There is also a model change which only includes adding `email` field to WPAccount since we already had `defaultBlog` relationship. Let me know if this is the correct way to do it.

Note: We already send a request for `email` field in MixPanel, I am not sure if I can just get rid of that thing all together or if we need to do something extra there (since we only update during login right now). Probably best to open up a new issue and ping @sendhil there just to make sure. Thoughts?
Since `createWordPressComAccountForUsername:password:authToken:` already creates an `AccountService` object you might want to move this code there and reuse it.