Support OpenJDK7

As a Stash administrator, I need to be able to install and run Stash using the JVM/JDK which ships with my Linux OS, so that I can reduce the administration and maintenance burden caused by needing to keep an additional external software package patched and up to date.

The default JVM/JDK which currently ships with many distributions (and will soon ship with most others) is OpenJDK 7 which is based on the same codebase as Oracle JDK7.