Jacoco reports aggregation mojo

Hi!

I've made a jacoco reports aggregation approach for multi-module projects. It relies on a set of assumptions:
 * plugin will not be used in parallel build (plugin invoked on last module in reactor with assumption that the rest of modules already have jacoco report generated)
 * jacaco reports already build on plugin execution

I also fount that it's not that easy to write a test for this plugin (maybe I have to use extensive mocking?). I'll write a test if someone point me in right direction :)

Here is an example plugin configuration:
```xml
<plugin>
    <groupId>org.eluder.coveralls</groupId>
    <artifactId>coveralls-maven-plugin</artifactId>
    <version>${coveralls.version}</version>
    <configuration>
        <skip>${skip.coveralls}</skip>
        <!-- relative path to module output directory  -->
        <coverageFile>jacoco/all/jacoco.xml</coverageFile>
    </configuration>
    <executions>
        <execution>
            <phase>site</phase>
            <goals>
                <goal>jacoco-aggregate</goal>
            </goals>
        </execution>
    </executions>
</plugin>
```