Maven support

Adds Maven support for the project. It allows us to manage dependencies more easily and, in the future, remove jar files in `lib/`.

### Commands

There are `mvn` commands corresponding to the ones that available by using `ant`:

* `mvn package` (=>`ant jar`)
* `mvn javadoc:javadoc` (=>`ant javadoc`)
* `mvn assembly:assembly` (=>`ant dist`)
 * You can build distribution package automatically by configuring `src.xml` and `bin.xml` under the `src/main/assembly`.

Currently `ant jnlp` is not supported (I found [webstart-maven-plugin](http://mojo.codehaus.org/webstart/webstart-maven-plugin/) but, sorry, I cannot configure it correctly...), but below is a workaround:

```
$ mvn package
$ ant jnlp-for-mvn
```

### Note
Currently the project's directory structure is not compatible with [Maven's standard layout](http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html). The recommended layout is below:

* `src/ow` => `src/main/java/ow`
* `src/resources` => `src/main/resources`
* `src/jnlp` => `src/main/jnlp`
* `test/ow` => `src/test/java/ow`

This patch coordinates them by using `build-helper-maven-plugin`.