Add 'order' method to query.

Manually apply diff from Tagtoo-master:

    5f796e66a189fe1cef466cdffe23896358b78dc9^..
    4a772c709fda1420a36e42bd0c90a0bb2fc791c5
Add coverage for Query.order().

Fix bug uncovered by those tests.
Neaten docstring.
Rationalize docstring.

Incorporate feedback from @dhermes.
Merge pull request #203 from tseaver/feature-datastore_query_order

Feature: datastore query order