Add support for querying by partition key alone

[jonterry-sans]

Requesting support for querying by just partition key: #11. This is a valid use case that would be leveraged when you split up a given entity over multiple items in a given partition to alleviate concerns about the 400kb item size limit.

e.g. You have a class record, and are concerned about the combination of students/teacher/TAs/assignments/tests/labs relationship data exceeding the limit of 400kb for a single item.

You can split all this data out in to multiple items with the same partition key and different sort keys. Then querying by just the partition key you can get all information regarding a given class while alleviating any concern about exceeding the 400kb item size limit.

[jeremydaly]

This would fall under #5, just adding query support.