Changed isAbsolute()-method of Folder class so it recognizes registered ...

...stream wrappers as absolute paths.

This has the benefit that the realpath() method is not applied to a registered stream wrapper in the constructor of the folder class. Using the realpath() method will break the stream.

This patch will make it possible to use vfsStream (a library for testing filesystem operations) together with cakephp apps

See also: http://stackoverflow.com/questions/24188008/is-it-possible-to-use-vfsstream-with-cakephp-2-4

I appreaciate all feedback!
Please follow cake's [coding standards](http://book.cakephp.org/2.0/en/contributing/cakephp-coding-conventions.html). Need a space after `if` and before `{`. Use can use the official [code-sniffer](http://github.com/cakephp/cakephp-codesniffer)