Adds CloudCode handler for beforeFind (#2715)

* Adds CloudCode handler for beforeFind

- Allows cloud code to modify a query before it is run
- Works with promises for a safer environment
- Supports modifiying the current query
- Supports issuing new queries

* Adds test for cornercase empty queries from rest

* Makes sure restOptions is always definied

src/rest.js

@@ -17,8 +17,12 @@ var triggers = require('./triggers');
 // Returns a promise for an object with optional keys 'results' and 'count'.
 function find(config, auth, className, restWhere, restOptions, clientSDK) {
   enforceRoleSecurity('find', className, auth);
-  let query = new RestQuery(config, auth, className, restWhere, restOptions, clientSDK);
-  return query.execute();
+  return triggers.maybeRunQueryTrigger(triggers.Types.beforeFind, className, restWhere, restOptions, config, auth).then((result) => {
+    restWhere = result.restWhere || restWhere;
+    restOptions = result.restOptions || restOptions;
+    let query = new RestQuery(config, auth, className, restWhere, restOptions, clientSDK);
+    return query.execute();
+  });
 }
 
 // get is just like find but only queries an objectId.