Zend Data Cache returns null when record doesn't exist (reverted from commit 590121c4)

- rename FileHelper::mkdir for API consistency
- changed default permission for directories to 775 instead of 777
- added some properties to classes that deal with files to allow control
  over directory permissions.

fixes #811

commit in redis branch was:
0cd65e74

use milliseconds only and fall back to seconds for older memcache
versions.

Moved add prefix ($this->keyPrefix) to the key in function buildKey(). Could it be better?

Fixed an example of using buildKey in the doc