Failing with UnsupportedCharsetExceptions

Hi there,

We're looking to use Trireme within our forthcoming [js-engine](https://github.com/typesafehub/js-engine#javascript-engine) based projects. Trireme looks great, and I've managed to get things working to a point.

When running a test within my IDE all is well. However if I run the test from the sbt shell i.e. outside of the IDE then I see:

    Caused by: java.nio.charset.UnsupportedCharsetException: Node-Base64
    	at java.nio.charset.Charset.forName(Charset.java:543)
    	at io.apigee.trireme.core.internal.Charsets.<clinit>(Charsets.java:39)

Here's how I'm calling Trireme (Scala/Akka code):

    val env = NodeEngine.nodePathEnv(userModulePaths)
    val nodeEnv = new NodeEnvironment()
    val sandbox = new Sandbox()
    sandbox.setStdin(stdinIs)
    sandbox.setStdout(stdoutOs)
    sandbox.setStderr(stderrOs)

    def receive = {
      case Execute =>

        if (log.isDebugEnabled) {
          log.debug("Invoking Trireme with {}", scriptArgs)
        }

        val script = nodeEnv.createScript(source.getName, source, scriptArgs)
        script.setSandbox(sandbox)
        script.setEnvironment(env.asJava)

        val senderSel = sender.path
        val senderSys = context.system
        val ec = script.execute().setListener(new ScriptStatusListener {
          def onComplete(script: NodeScript, status: ScriptStatus): Unit = {
            stdoutOs.close()
            stderrOs.close()
            senderSys.actorSelection(senderSel) ! status.getExitCode
          }
        })  
    }

trireme-core is my one and only dependency.

Any quick fixes?