Merged branch frontend
Merge branch 'frontend'
Fix.  Add logging library.
Fix.  Minor indentation issues.
Fix. If any key-word argument is empty, set it to default.
Add. Store a class variable that contains a default time range for metric objects `DEFAULT_DATA_RANGE`
Add. logger + docs.
Mod. moved `date_start` & `date_end` parameter definitions into user metric super class.  These will apply to all metrics.
Fix. Remove reference to setting datetime defaults outside of UserMetric class.
Add. `date_start` and `date_end` properties to metric classes.  Utilize this in metrics_manager module rather than handling timestamps
Fix. Order of args to get_agg_key()
Mod. Move tool method build_key_tree() into engine.py.
Fix. If the retrieval of the cohort id "last updated" timestamp  raises an exception pass without the value.
Fix. Removed old print statements for emitting output.
Fix. Representation of 'aggregator' to be independent of metric.
Merged frontend
Copy of homepage and single metric page
Mod.  Let get_url_from_keys() take an optional path root to compose request urls.
Add. New view to process and return stored responses.
Fix. Add `project` and `namespace` ro RequestMeta model.