Added Dockerfile

Added a `Dockerfile` so that `htmlemail` can be used straight-away without dealing with dependencies.

>Docker is an open platform for developers and sysadmins to build, ship, and run distributed applications.

To build it : 

```
docker build -t htmlemail .
docker run -t -i htmlemail yo htmlemail
```

To test it without building it :

```bash
docker pull zboralski/htmlemail
docker run -t -i zboralski/htmlemail yo htmlemail
```