Enabled set url prefix

I added prefix option to set sub-url when you run standalone jar.
You can set context-path in a similar way to set port.

If you run standalone jar with below command,

```
java -jar -Dskinny.prefix=sub-url standalone-build/target/scala-2.10/skinny-standalone-app-assembly-0.1.0-SNAPSHOT.jar
```

application top is below url.

```
http://localhost/sub-url/
```