Adding sort_keys

[eudoroolivares2016]

Minor spelling fixes

adds sort keys for granules will do collections some other time

testing:
start up python in the repo

from cmr import CollectionQuery, GranuleQuery, ToolQuery, ServiceQuery, VariableQuery
import json
api = GranuleQuery()
api.parameters(short_name="OMNO2", version="003", provider='GES_DISC', sort_key='-start_date')
granules = api.get(10)`

print(json.dumps(granules, indent=1))

ensure that the returned granules are sorted by the sort_key

[briannapagan]

We're in need of this feature at GES DISC - I had put in a request to be maintainer to help approve PRs months ago for python-cmr but it was never approved. Can I nudge @frankinspace to review/approve?

[frankinspace]

@eudoroolivares2016 Can you please add an entry to the changelog (under the Unreleased header create a Added section and describe the new feature). Also, can you add a note in the readme about sort keys.

Thank you for the contribution!