Add cursor handling to datastore.query.Query.

Manually apply diff from Tagtoo-master:

    25672e10fc359c67e0d267db2f23928b0a5761ca^..
    90ac07db1bcd3ebfd89f43ee9b59ded1cd8dd643

Add unit test, fix bugs.
Fix typo.

Incoporate feedback from @dhermes.
Resolve conflict with master after 'order' feature landed.
Merge pull request #204 from tseaver/feature-datastore_query_cursor

Add cursor handling to datastore.query.Query.