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}`.

Perhaps its intentional, but someone could set `MAVEN_SETTINGS_OPT` directly and stick whatever they want into the `BUILDCMD`. Maybe add an `else` block above to unset `MAVEN_SETTINGS_OPT`?