Added support for a custom settings.xml.

I went with the simplest approach for getting a custom `settings.xml` into the slug. I did not include http://s3.amazonaws.com/heroku-jvm-langpack-java/settings.xml as it did not seem necessary. 

I also did not include `settings-security.xml`. Instead, users can create config vars with secrets such as passwords or URLs. These can then be used in the settings.xml as `${env.MVN_PRIVATE_REPO_PASSWORD}`.

There are a couple considerations. `settings.xml` can have secrets, so some users *may* not want to check the file into their git repo. In that case, a URL might make more sense. However, I think the better approach is to check `settings.xml` into your app and use environment variables to configure secrets (e.g. ${env.MAVEN_PASSWORD}). The sane default location is either the root of the project, or resources, but it probably makes sense to allow a `$MAVEN_SETTINGS_PATH` relative to the app root.