Add file/directory existance checks to grunt

Add functions to simply check if a file and/or a directory exists.

Example:
var c5fs = require('libraries/fs');
var isDirectory = c5fs.isDirectory(path);
var isFile = c5fs.isFile(path);
var isFileOrDirectory = c5fs.fileExists(path);
Merge pull request #1620 from mlocati/add-useful-grunt-functions

Add file/directory existance checks to grunt