Add support testing email function in gae

[alexlehm]

I just wrote a mail handler for my GAE application, which worked quite well, but was difficult to debug since the function is not available in the dev server.

The python gae development server has a function to send test emails to the server running locally, when you call localhost:port/_ah/admin/ you can create a sample email to test the handler.

I'm not sure if a similar admin function is available in the java version of gae as well, if it is, it would be nice if the admin pages could be exposed as a route in play, if not, it would be possible to write a mail test function that creates a sample email and post it to the handler directly (in fact I could write that if you like)

[alexlehm]

I found the description how to use the test mail function in the java server, however this only works when using a regular war directory, it doesn't work with play dev mode, since the urls are not available.