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/2560/5371177/c68b1778-8085-11e4-9b6d-8d1b223a5ae1.png)

As a bonus, the daemon will also start when you `heroku run bash # or a Rails console or anything else`.

Context: [DevCenter article](https://t.co/RxIrEaSnSV) and [tweet](https://twitter.com/bjeanes/status/542534741946159105)