Add simple depends_on API.

recipes/next_prime.py

+from recipe_engine.recipe_api import Property
+from recipe_engine import config
+import math
+
+DEPS = [
+    'recipe_engine/properties',
+]
+
+PROPERTIES = {
+    'curr_prime': Property(kind=int),
+}
+
+RETURN_SCHEMA = config.ReturnSchema(
+    result=config.Single(int),
+)
+
+def is_prime(val):
+  if val % 2 == 0:
+    return False
+  for div in range(3, math.ceil(math.sqrt(val))):
+    if val % div == 0:
+      return False
+  return True
+
+def RunSteps(api, curr_prime):
+  curr_prime += 2
+  while not is_prime(curr_prime):
+    curr_prime += 2
+
+  return RETURN_SCHEMA(result=curr_prime)
+
+def GenTests(api):
+  yield (
+      api.test('basic') +
+      api.properties(curr_prime=3)
+  )