Prerequisite changes to allow Java buildpack support

Hi,
These commits add:
 * sourcing of /app/.profile.d scripts to allow dependencies like Java to be activated properly for the java buildpack to work
 * allowing a developer to override repository/script locations when running the bootstrap.sh script using environment variables. Your repo locations remain the defaults.

I've tested using forks of the Java and Ruby samples from Heroku (https://github.com/rnorth/java-sample and https://github.com/rnorth/ruby-sample) - the ruby sample needed some dependencies to be updated.

Hope this is OK!

Richard