fix #1139: call RefreshBlogContentTask on first blog when user login

fix #1139 

* call RefreshBlogContentTask on first blog when user logins
* call RefreshBlogContentTask when user adds a self hosted blog
* call RefreshBlogContentTask on blog changed
* remove calls to RefreshBlogContentTask in PostsActivity
According to [the AsyncTask docs](http://developer.android.com/reference/android/os/AsyncTask.html#executeOnExecutor%28java.util.concurrent.Executor,%20Params...%29), executeOnExecutor must be called from the main thread, but it won't be here since this is called from SetupBlogTask.doInBackground.