Add getRootHash and getCommitObject on project API

[pmeijer]

Before these you needed to first obtain the commit-hash and load the commit-object using the quasi-internal and non-promisfied method loadObject. These simplify the loading of a new root in the core given e.g. a branch:

project.getRootHash('master')
  .then(rootHash => core.loadRoot(rootHash))   
  .then((rootNode) => {
     ...
  })

        /**
         * Retrieves the root hash at the provided branch or commit-hash.
         * @param {string} branchNameOrCommitHash - Name of branch or a commit-hash.
         * @param {function} [callback] - if provided no promise will be returned.
         *
         * @return {external:Promise}  On success the promise will be resolved with
         * {@link module:Core~ObjectHash} <b>rootHash</b>.<br>
         * On error the promise will be rejected with {@link Error} <b>error</b>.
         */
        this.getRootHash = function (branchNameOrCommitHash, callback) {
        ...
        }

        /**
         * Retrieves the commit-object at the provided branch or commit-hash.
         * @param {string} branchNameOrCommitHash - Name of branch or a commit-hash.
         * @param {function} [callback] - if provided no promise will be returned.
         *
         * @return {external:Promise}  On success the promise will be resolved with
         * {@link module:Storage~CommitObject} <b>commitObject</b>.<br>
         * On error the promise will be rejected with {@link Error} <b>error</b>.
         */
        this.getCommitObject = function (branchNameOrCommitHash, callback) {
        ...
        }