Create standard Sprint queries.

The intention is to use one of these queries as the basis for any
sprint query that is needed in the rest of the system.  These all
return an IQueriable<Sprint> so they can be modified further.

The consumer is responsible for the select clause.
Add work item queries and filters.

These are to be used where the work items are to be retrieved in a
standard way.  Currently, this is only in the work items controller.

The Sprint conttroller gets work items as well, but uses a non-standard
sorting.
Merge branch 'feature/QueryExtensions' into develop