Refactoring for style

Hello, I made a couple of changes.
I believe they are all backwards compatible as it's all style..

* add `license_key` meta attribute that is then set to both current app and server monitoring attributes.
This gives typical users one smaller target but keeps the same flexibility as app and server can be defined separately.

e.g. 
```json
{
  "default_attributes": {
        "newrelic": {
          "license_key": "123456789123456789",
          "web_server": { "service_name": "nginx" }
  }
}
```

Rather than:
```json
{
  "default_attributes": {
        "newrelic": {
          "server_monitoring": {
            "license_key": "123456789123456789"
          },
          "application_monitoring": {
             "license_key": "123456789123456789"
          }, 
          "web_server": { "service_name": "nginx" }
  }
}
```

* Use apt_repository LWRP for installing on debian variants - this was a large removal of code. It added a lot of complexity when a [builtin LWRP](http://docs.opscode.com/lwrp_apt.html#apt-repository) can be used instead.
This should be faster and more robust.

* Case statement for php-agent startup mode. The logic for this did not work for me.
The case statement does.

* A number of [Foodcritic](http://acrmp.github.io/foodcritic/) lint checks did not pass. I mainly changed all attribute naming styles to be consistent, which is the big code churn in the diffs.

* I took the liberty of adding to the README and version bumping.