Use \OC\Preferences::getValueForUsers() in EmailNotification

Fix #17
Add a test for Hooks::filterUsersBySetting()

Fix #17
Use \OC\Preferences::getValueForUsers() in Hooks::filterUsersBySetting()
Add test for method = 'email'
Replace query and aget batchtime with \OC\Preferences
Add a user that has no batchtime setting
Remove unnecessary parameter from tests
Merge pull request #31 from owncloud/issue/17

[issue/17] Use \OC\Preferences::getValueForUsers() instead of custom queries