add support for QuerySet.explain()

[timgraham]

Example output:

>>> print(Question.objects.explain())
explainVersion: '1'
queryPlanner: {   'indexFilterSet': False,
    'maxIndexedAndSolutionsReached': False,
    'maxIndexedOrSolutionsReached': False,
    'maxScansToExplodeReached': False,
    'namespace': 'mysite.polls_question',
    'optimizedPipeline': True,
    'parsedQuery': {},
    'rejectedPlans': [],
    'winningPlan': {'stage': 'EOF'}}
executionStats: {   'allPlansExecution': [],
    'executionStages': {   'advanced': 0,
                           'executionTimeMillisEstimate': 0,
                           'isEOF': 1,
                           'nReturned': 0,
                           'needTime': 0,
                           'needYield': 0,
                           'restoreState': 0,
                           'saveState': 0,
                           'stage': 'EOF',
                           'works': 1},
    'executionSuccess': True,
    'executionTimeMillis': 0,
    'nReturned': 0,
    'totalDocsExamined': 0,
    'totalKeysExamined': 0}
command: {   '$db': 'mysite',
    'aggregate': 'polls_question',
    'cursor': {},
    'pipeline': [{'$match': {'$expr': {}}}]}
serverInfo: {   'gitVersion': 'b6513ce0781db6818e24619e8a461eae90bc94fc',
    'host': 'westbrook',
    'port': 27017,
    'version': '7.0.12'}
serverParameters: {   'internalDocumentSourceGroupMaxMemoryBytes': 104857600,
    'internalDocumentSourceSetWindowFieldsMaxMemoryBytes': 104857600,
    'internalLookupStageIntermediateDocumentMaxSizeBytes': 104857600,
    'internalQueryFacetBufferSizeBytes': 104857600,
    'internalQueryFacetMaxOutputDocSizeBytes': 104857600,
    'internalQueryFrameworkControl': 'trySbeRestricted',
    'internalQueryMaxAddToSetBytes': 104857600,
    'internalQueryMaxBlockingSortMemoryUsageBytes': 104857600,
    'internalQueryProhibitBlockingMergeOnMongoS': 0}

Fixes #28

[WaVEV]

kwargs will only have a field or zero. could verbosity be passed in a different way to database.command?

[timgraham]

An alternative would be verbosity = options.get("verbosity") and change the **kwargs line to **{"verbosity": verbosity} if verbosity else {}.

The current approach seems more extensible. For example, there's a comment kwarg that I didn't implement.

[timgraham]

It was trivial to support comment by adding a loop.