maven-plugin: Generate vocabularies from remote URLs

With this change it is possible to generate a vocabulary using a URL:

```xml
<plugin>
	<groupId>com.github.tkurz</groupId>
	<artifactId>sesame-vocab-builder-maven-plugin</artifactId>
	<version>1.1-SNAPSHOT</version>
	<executions>
		<execution>
			<id>generate-vocabularies</id>
			<phase>generate-sources</phase>
			<goals>
				<goal>generate</goal>
			</goals>
			<configuration>
				<vocabularies>
					<vocabulary>
						<className>SKOS</className>
						<url>http://www.w3.org/2004/02/skos/core#</url>
					</vocabulary>
				</vocabularies>
			</configuration>
		</execution>
	</executions>
</plugin>
```

I think it is time for a relesase ;-)