Fixed crash in iOS 6 native twitter authorization

ACAccountType gets released after ACAccountStore is released(ACAccountType is owned by ACAccountStore) - ACAccount has invalid accountType info. 
This causes 'Invalid account type for this request' crash in iOS 6.
That's why we need to keep reference to ACAccountStore.

Please look at http://stackoverflow.com/questions/13349187/strange-behaviour-when-trying-to-use-twitter-acaccount