Implement utility function for loading a context.

[pmeijer]

/**
 * Opens the context specified by the parameters and returns a result object containing
 * data.
 * @param {object} storage - clientstorage, serveruserstorage or localstorage
 * @param {object} gmeConfig - global webgme configuration
 * @param {object} parameters
 * @param {string} [parameters.projectName] - name of project to open -> result.project
 * @param {string} [parameters.branchName] - name of branch to load root from. -> result.rootNode, result.commitHash
 * @param {string} [parameters.commitHash] - if branchName not given commitHash will be loaded. -> result.rootNode
 * @param {string} [parameters.nodeIds] - loads all specified node ids. -> result.nodes
 * @param {boolean} [parameters.meta] - loads all META-nodes. -> result.META
 * @param {object} [parameters.core] - Used if branchName or commitHash is specified (a new Core will be created
 *                                     if needed and not provided here). -> result.core
 * @param {function} callback
 */

var openContext = function (storage, gmeConfig, parameters, callback);