Created custom Play runner starter for New Relic

Created a custom runner to run a Play app with the New Relic agent attached.  I did clobber some stuff from the default NR plugin you may want to add back, not sure if/how it's used.  To use the new plugin the only thing you need to do is set:

```
set newRelicAgentJar := "<someplace>/newrelic.jar"
set newRelicConfigFile := "<someplace>/newrelic.yml"
```

The question becomes how/where to set this.  The natural place is in the injected shim file when the project is enabled.

For review by @henrikengstrom, @havocp, and @jsuereth 
Does the removal of these keys from this context mean that it shouldn't be able to run ``backgroundRun`` in the SbtNrPlugin context anymore?