Overriding the parent class designated initializer

Changed: ```FMDatabase```, ```FMDatabasePool``` and ```FMDatabaseQueue```.

This ensures that the class will be fully initialized even if using the designated initializer of NSObject.
Also, provides a nice default for the ```path```: in memory database.

This change also makes it a little bit easier to integrate ```FMDB``` with https://github.com/atomicobject/objection.

Documentation: https://developer.apple.com/library/ios/documentation/general/conceptual/devpedia-cocoacore/MultipleInitializers.html