Added Less support

[koen-serry]

Added Less support
Haml acts now the same way as less/coffee do
Avoid compiling twice

[tjerkw]

I would argue using a separate less plugin like this:

            <!-- This plugin performs less css handling -->
            <plugin>
                <groupId>org.lesscss</groupId>
                <artifactId>lesscss-maven-plugin</artifactId>
                <version>1.3.0</version>
                <configuration>
                    <sourceDirectory>${project.basedir}/src/main/webapp/css</sourceDirectory>
                    <outputDirectory>${project.build.directory}/${project.build.finalName}/css</outputDirectory>
                    <compress>true</compress>
                    <includes>
                        <include>*.less</include>
                    </includes>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>