Add: repository site prepare goal to copy eclipse repository resources

This change copies p2 repository files into the sites folder so that they can deployed with site files as part of a "mvn site-deploy" execution.

For example, you can add the following XML to your eclipse-repository project so that you can upload your p2 repository to your Github pages:

	<distributionManagement>
		<site>
			<id>github-project-site</id>
			<url>gitsite:git@github.com/rgladwell/tycho-demo.git/updates</url>
		</site>
	</distributionManagement>
	<build>
		<extensions>
			<extension>
				<groupId>org.apache.maven.scm</groupId>
				<artifactId>maven-scm-provider-gitexe</artifactId>
				<version>1.3</version>
			</extension>
			<extension>
				<groupId>org.apache.maven.scm</groupId>
				<artifactId>maven-scm-manager-plexus</artifactId>
				<version>1.3</version>
			</extension>
			<extension>
				<groupId>org.kathrynhuxtable.maven.wagon</groupId>
				<artifactId>wagon-gitsite</artifactId>
				<version>0.3.1</version>
			</extension>
		</extensions>
	</build>

Please see the tycho-demo project for an example of project testing this change:

https://github.com/rgladwell/tycho-demo