Move dogstatsd start into .profile.d script

This should skip needing users of the buildpack to add the extra level of indirection by using a second `Procfile`. Tested my changes and was successfully getting metrics: 

![metric explorer datadog 2014-12-10 15-55-23](https://cloud.githubusercontent.com/assets/2560/5371137/f8940e92-8084-11e4-8154-5db4ce77d132.png)

This might not strictly be necessary. I don't remember offhand if we execute these scripts or source them, but it may likely be the latter.