Adding in Second and Minute time grains for MySQL (#696)

* Added documentation of the health check endpoint

* Adding Minute and Second time grains for MySQL

* Fixed Migration script so that it doesn't break when new fields added to the models for Dashboards or Slices (using declarative base)

* Revert "Fixed Migration script so that it doesn't break when new fields added to the models for Dashboards or Slices (using declarative base)"

This reverts commit 0b6dd69.

* Code Cleanliness

caravel/models.py

@@ -414,6 +414,11 @@ def grains(self):
             ),
             'mysql': (
                 Grain('Time Column', _('Time Column'), '{col}'),
+                Grain("second", _('second'), "DATE_ADD(DATE({col}), "
+                      "INTERVAL (HOUR({col})*60*60 + MINUTE({col})*60"
+                      " + SECOND({col})) SECOND)"),
+                Grain("minute", _('minute'), "DATE_ADD(DATE({col}), "
+                      "INTERVAL (HOUR({col})*60 + MINUTE({col})) MINUTE)"),
                 Grain("hour", _('hour'), "DATE_ADD(DATE({col}), "
                       "INTERVAL HOUR({col}) HOUR)"),
                 Grain('day', _('day'), 'DATE({col})'),