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)
See L17. It wraps the export and daemon start in a subshell `()` so that the env vars don't leak out. 

``` sh
$ heroku run -a bo-datadog-buildpack-test -- "env | grep LD_"
Running `env | grep LD_` attached to terminal... up, run.1533
Log file is unwritable: '/var/log/datadog/dogstatsd.log'
2014-12-10 23:05:28,465 | INFO | dd.dogstatsd | daemon(daemon.py:154) | Starting daemon
2014-12-10 23:05:28,466 | INFO | dd.dogstatsd | daemon(daemon.py:163) | Daemon pidfile: /tmp/dogstatsd.pid
```

There's some noise of the daemon starting up, but no `LD_LIBRARY_PATH` exported.