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}`.

No good reason. I included this because it was in @jsimone's custom_settings branch. but he was defaulting to https://s3.amazonaws.com/heroku-jvm-langpack-java/settings.xml -- so this probably isn't important anymore.