Merge remote-tracking branch 'mikeperry-github/bug29089'

changes/ticket28142

+  o Major features (circuit padding):
+    - Implement preliminary support for the circuit padding portion of
+      Proposal 254. The implementation supports Adaptive Padding (aka
+      WTF-PAD) state machines for use between experimental clients and
+      relays. Support is also provided for APE-style state machines that
+      use probability distributions instead of histograms to specify
+      inter-packet delay. At the moment, Tor does not provide any padding
+      state machines that are used in normal operation -- this feature
+      exists solely for experimentation in this release. Closes
+      ticket 28142.