Implement support for SSM parameter store

This commit adds initial support for the Simple System Manager client.

It currently only mocks the following api endpoints:
 - delete_parameter()
 - put_parameter()
 - get_parameters()

moto/ssm/urls.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+from .responses import SimpleSystemManagerResponse
+
+url_bases = [
+    "https?://ssm.(.+).amazonaws.com",
+]
+
+url_paths = {
+    '{0}/$': SimpleSystemManagerResponse.dispatch,
+}