Fix notification click behaviour for multiple accounts.

Test case:
- Have two accounts A and B
- Get a mail on A
- Get a mail on B
- Click on the notification for A

Result:
You end up seeing the message from B instead of A

The reason for that is that we need to register separate PendingIntents
for each account.
Merge pull request #225 from maniac103/fix-multiaccount-notification

Fix notification click behaviour for multiple accounts.