Add plugin support to skydock

Plugins allow you to easily write javascript functions to modify how skydock works.  Here is the example from the readme.

#### Plugin support
I just added plugin support via [otto](https://github.com/robertkrimen/otto) to allow users to write plugins in javascript.  Currently only one function uses plugins and that is `createService(container)`.  This function takes a container's configuration and converts it into a DNS service  The current functionality is implementing in this javascript function:

```javascript
function createService(container) {
    return {
        Port: 80,
        Environment: defaultEnvironment,
        TTL: defaultTTL,
        Service: cleanImageName(container.Image),
        Instance: removeSlash(container.Name),
        Host: container.NetworkSettings.IpAddress
    }; 
}
```

Your function must be called `createservice` which takes one object, the container, and must return a service with the fields shown above.  In your plugin you have access to the following global variables and functions.


```javascript
var defaultEnvironment = "string - the environment from the -environment flag";
var defaultTTL = 30; // int - the ttl value from the -ttl flag

function cleanImageName(string) string // cleans the repo and tags of the passed parameter returning the result
function removeSlash(string) string  // removes all / from the passed parameter returning the result
```

And that is it.  Just add a `createservice` function to a .js file then use the `-plugins` flag to enable your new plugin.  Plugins are loaded at start so changes made to the functions during the life of skydock are not reflected, you have to restart ( done for performance ).  

```bash
docker run -d -v /var/run/docker.sock:/docker.sock -v /myplugins.js:/myplugins.js -name skydock -link skydns:skydns crosbymichael/skydock -s /docker.sock -domain docker -plugins /myplugins.js
```

Feel free to submit your plugins to this repo under the `plugins/` directory.