Code to make clients fetch and use microdescriptors for circuit building

To turn this on, set UseMicrodescriptors to "1" (or "auto" if you
want it on-if-you're-a-client).  It should go auto-by-default once
0.2.3.1-alpha is released.

Because of our node logic, directory caches will never use
microdescriptors when they have the right routerinfo available.

changes/microdesc_use

@@ -0,0 +1,10 @@
+  o Major features
+    - Clients can now use microdescriptors instead of regular descriptors
+      to build circuits.  Microdescriptors are authority-generated and
+      -authenticated summaries of regular descriptors' contents, designed
+      to change very rarely.  This feature is designed to save bandwidth,
+      especially for clients on slow internet connections.  It's off
+      by default for now, since nearly no caches support it, but it will
+      be on-by-default for clients in a future version.  You can use the
+      UseMicrodescriptors option to turn it on.
+