feat(datastore): move Dataset.key() arguments to array

Specify parameters in Dataset.key() in an array instead of as a list
of arguments. Single parameter can be specified sans in an array.

BREAKING CHANGE: Dataset.key() no longer accepts arguments list of
parameters and no longer requires null parameters for an unknown
identifier when creating incomplete keys. Single kind can still be
specified without surrounding in an array.

To migrate the code, simply surround arguments in an array and
remove the trailing null if you have it. See the example below.

Before:

var key1 = dataset.key('Company', null);
var key2 = dataset.key('kind1', 123, 'kind2', 'id2');
var key3 = dataset.key('kind1', 123, 'kind2', null);

After:

var key1 = dataset.key('Company');
var key2 = dataset.key(['kind1', 123, 'kind2', 'id2']);
var key3 = dataset.key(['kind1', 123, 'kind2']);
Merge pull request #209 from ryanseys/key-docs

Dataset.key() docs + proposal for simpler API