Use the `java.home` property instead of `jdk.home`.
New location for DocErrorReporter in JDK 1.7; better handle when missing.

Use `System.exit(1)` to ensure that errors are reported via exit code
even when we can't find an appropriate DocErrorReporter.  Also work
harder to find the right DocErrorReporter for JDK 1.4 and 1.7.  This
ensures the JDoctest doesn't silently let failing doctests through.
Add travis test integration.