Possiblity to mock files/directories and file content

Added possibility to mock files/directories as well as file content.  This makes it possible to test cases where one expect a file to exist on the device. Earlier this was not possible as the readFile/readAsText always would return undefined, this would break any app that expects a file to actually have content. 

This PR should be backward compatible as all new functionality needs "shouldMockFiles" flag to be set(this is false by default).