Proxy method can be specified via arguments

The previously introduced proxy feature can be configured by setting the
`APIMethod` and `proxyURL` properties in the `express-server` options.

This change makes it possible to quickly change the used proxy method by
specifying an argument to the `server` and `test:server` task: to use
the `stub` proxy method, run `grunt server:stub` or `grunt
test:server:stub` and to proxy the requests to a proxy server, invoke
`grunt server:proxy` or `grunt test:server:proxy`.

If no proxy method is specified (aka `grunt:server`), then the proxy
configuration from the `express-server` options is used.

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This feature is inspired by @ralfschimmel's [comment](https://github.com/stefanpenner/ember-app-kit/pull/304#issuecomment-29445844).