Allow specifying which fields to return

Tests included! Currently only for /runs/ and /runs/latest/, not /runs/$NAME
Add field-querying for jobs

With lots of tests! API is /runs/$run/jobs/?fields=field1,field2
Currently not implemented for single jobs.
Don't limit jobs returned by /runs/foo/jobs/ to 10
Update latest_runs() to reflect timestamp rename
Make latest_runs() return a simple list
Move slice() to Base class