Incompatibility with PHP7

[dpoposki]

Under PHP7 environment, the PHT\Cache\Void and PHT\Log\Void are not valid class names.

Fatal error: Cannot use 'Void' as class name as it is reserved in PHT/Log/Void.php on line 16

What I would suggest here is that we rename the classes to something like VoidCache and VoidLogger, although we will introduce a naming inconsistency in the Cache and Log classes. Another suggestion would be to rename all of the classes, i.e. VoidLogger, FileLogger.

[jetwitaussi]

Rename classes to None