resources: Add poll metaparameter

This allows a resource to use polling instead of the event based
mechanism. This isn't recommended, but it could be useful, and it was
certainly fun to code!

examples/poll1.yaml

@@ -0,0 +1,24 @@
+---
+graph: mygraph
+resources:
+  file:
+  - name: file1
+    meta:
+      poll: 5
+    path: "/tmp/mgmt/f1"
+    content: |
+      i am f1
+    state: exists
+  - name: file2
+    path: "/tmp/mgmt/f2"
+    content: |
+      i am f2
+    state: exists
+  - name: file3
+    meta:
+      poll: 1
+    path: "/tmp/mgmt/f3"
+    content: |
+      i am f3
+    state: exists
+edges: []