Fix AppEngine builds

`go build -tags appengine [...]` currently yields:

```
.../github.com/sirupsen/logrus/text_formatter.go:84:4: undefined: initTerminal
```

This change implements `initTerminal` for AppEngine builds.

terminal_appengine.go

@@ -0,0 +1,11 @@
+// Based on ssh/terminal:
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appengine
+
+package logrus
+
+func initTerminal(w io.Writer) {
+}