add screenshot utilities

[themouette]

For easy saving

module.exports.register = function (Driver, config) {
    var path = require('path');
    var fs = require('fs');

    function readKey(obj,i) {return obj && i in obj ? obj[i] : null; }
    var destDir = 'screenstory.storage'.split('.').reduce(readKey, config);
    destDir || (destDir = path.resolve(process.cwd(), "screenshots"));
    createFullPath(destDir, function (err) {
                if (err) console.log(err);
                else console.log('"%s" created to store screenshots', destDir);
            });

    Driver.prototype.thenScreenstory = function (id) {
        return this.thenTakeScreenshot(function (image, next) {
            saveScreenshot(id, image, this._desired, next);
        });
    };

    function saveScreenshot(id, b64screen, desired, next) {
        var file = filename(id, desired);
        fs.writeFile(file, b64screen, 'base64', function (err) {
            next(err);
        });
    }

    function filename(id, desired) {
        return path.resolve(destDir, id+'_'+desired.browserName+'.png');
    }

    // thx to https://gist.github.com/aelaguiz/864454
    function createFullPath(fullPath, callback) {
        var parts = path.normalize(fullPath).split("/"),
        working = '/',
        pathList = [];

        for(var i = 0, max = parts.length; i < max; i++) {
            working = path.join(working, parts[i]);

            pathList.push(working);
        }

        var recursePathList = function recursePathList(paths) {
            if(0 === paths.length) {
                callback(null);
                return ;
            }

            var working = paths.shift();

            try {
                fs.exists(working, function(exists) {
                    if(!exists) {
                        try {
                            fs.mkdir(working, 0755, function() {
                                recursePathList(paths);
                            });
                        }
                        catch(e) {
                            callback(new Error("Failed to create path: " + working + " with " + e.toString()));
                        }
                    }
                    else {
                        recursePathList(paths);
                    }
                });
            }
            catch(e) {
                callback(new Error("Invalid path specified: " + working));
            }
        };

        if(0 === pathList.length)
            callback(new Error("Path list was empty"));
        else
            recursePathList(pathList);
    }
};