Add tower server example (#375)

Note that this also adds `impl From<Status> for http::Response<BoxBody>`
in order to make writing middleware which can return an error much
easier.

examples/Cargo.toml

@@ -66,6 +66,10 @@ path = "src/tls_client_auth/server.rs"
 name = "tls-client-auth-client"
 path = "src/tls_client_auth/client.rs"
 
+[[bin]]
+name = "tower-server"
+path = "src/tower/server.rs"
+
 [[bin]]
 name = "multiplex-server"
 path = "src/multiplex/server.rs"