Add serializer option to Memcached Cache engine

Add option to select a serializer engine among php, json or igbinary
Throw exception when selected serializer is not installed
Remove unnecessary sprintf
Fix test
Rename `serializer` to `serialize`
Add doc about the new serialize setting
Change $serializer visibility, and other fix
Improve serializer engine assignment code
Merge pull request #1654 from kamisama/improvement/2.5-memcached-cache-engine-fix

Add option to select a serializer engine in Memcached Cache engine