Gem support

This pull request adds support for loading gems, which is useful for Sass/Compass extensions. Download a gem into target/rubygems (the default), e.g. using the gem-maven-plugin. Then the extensions can be loaded by setting the following in the plugin config.

    ...
    <configuration>
      <gems>
        <gem>zen-grid</gem>
      </gems>
      <gemPaths>
        <gemPath>${project.build.directory}/rubygems</gemPath>
      </gemPaths>
    </configuration>
    ...