backend support for live stream (#59)

This is so far completely untested, for use by a new UI prototype.

It creates a new URL endpoint which sends one video/mp4 media segment
per key frame, with the dependent frames included. This means there will
be about one key frame interval of latency (typically about a second).
This seems hard to avoid, as mentioned in issue #59.

base/error.rs

@@ -40,6 +40,10 @@ impl Error {
     pub fn kind(&self) -> ErrorKind {
         *self.inner.get_context()
     }
+
+    pub fn compat(self) -> failure::Compat<Context<ErrorKind>> {
+        self.inner.compat()
+    }
 }
 
 impl Fail for Error {