Sign out

Add a Account Info section to Settings menu for displaying user name, space usage, space total size and sign out current account option.

Since space usage may change frequently, this PR used Service to fetch the lasted usage data from server.
The way of listing accounts in listview of AccountsActivity doesn\`t depends on token, that\`s to say, even though I set token to null in database and shared prefs doesn\`t stop the AccountsActivity from showing the specific account in its listview. 

It doesn\`t make any sense to show the already sign out account in AccountsActivity. As a trade off I deleted almost everything of the sign out account in order to remove the account from the listview of AccountsActivity.

I know that\`s not the best choice, I need to refactor the code again (needs a lot of code modification) to make token works as expected.