Vendor maven by version

This PR would allow a user to specify the desired version of Maven in the `system.properties` file like so:

```
java.runtime.version=1.7
maven.version=3.2.3
```

It also moves the Maven package builder scripts into the `vendor/maven/` dir so that buildpack users can submit new version as PRs against `vendor/maven/sources.txt`. In this future, this might be enhanced to download Maven at compile time if the version is not pre-packaged. But it doesn't do that now.