Doubled javaagent in argLine

1. I'm using plugin 0.6.0
2. I've jacoco-maven-plugin doubled because I want to measure unit tests result (in test phase with surefire) but also integration tests (in integration-test phase with failsafe)
3. Before test phase everything is OK:
```argLine set to -javaagent:/home/devel/tools/maven/repo/org/jacoco/org.jacoco.agent/0.6.0.201210061924/org.jacoco.agent-0.6.0.201210061924-runtime.jar=destfile=/home/devel/projects/micro/webapp/target/jacoco-unit.exec```
4. Before integration-test phase there is problem with correct argLine, because of the prepending it looks that:
```argLine set to -javaagent:/home/devel/tools/maven/repo/org/jacoco/org.jacoco.agent/0.6.0.201210061924/org.jacoco.agent-0.6.0.201210061924-runtime.jar=destfile=/home/devel/projects/micro/webapp/../target/jacoco-it.exec -javaagent:/home/devel/tools/maven/repo/org/jacoco/org.jacoco.agent/0.6.0.201210061924/org.jacoco.agent-0.6.0.201210061924-runtime.jar=destfile=/home/devel/projects/micro/webapp/target/jacoco-unit.exec```
As you can see there are two javaagents what results in:
```java.lang.instrument.IllegalClassFormatException: Error while instrumenting class org/apache/maven/surefire/booter/ForkedBooter.
Caused by: java.lang.IllegalStateException: Class org/apache/maven/surefire/booter/ForkedBooter is already instrumented.```
When we remove second javaagent (by defining it in the argLine property in the failsafe-plugin configuration it works great. 

Example pom's:
```
 <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>2.12.4</version>
        <configuration>
          <excludedGroups>gui,arquillian</excludedGroups>
        </configuration>
      </plugin>

      <plugin>
        <groupId>org.jacoco</groupId>
        <artifactId>jacoco-maven-plugin</artifactId>
        <version>${jacoco.version}</version>
        <executions>
          <execution>
            <id>jacoco-initialize</id>
            <goals>
              <goal>prepare-agent</goal>
            </goals>
            <configuration>
              <destFile>${project.basedir}/target/jacoco-unit.exec</destFile>
            </configuration>
          </execution>
        </executions>
      </plugin>

    </plugins>
  </build>

  <profile>
      <id>arquillian</id>
       <build>
        <plugins>
          <plugin>
            <groupId>org.jacoco</groupId>
            <artifactId>jacoco-maven-plugin</artifactId>
            <version>${jacoco.version}</version>
            <executions>
              <execution>
                <id>pre-integration-test</id>
                <goals>
                  <goal>prepare-agent</goal>
                </goals>
                <phase>pre-integration-test</phase>
                <configuration>
                  <destFile>${project.basedir}/../target/jacoco-it.exec</destFile>
                </configuration>
              </execution>
            </executions>
          </plugin>
          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-failsafe-plugin</artifactId>
            <version>2.12.4</version>
            <configuration>
              <groups>gui,arquillian</groups>         
              <includes>
                <include>**/*.java</include>
              </includes>
            </configuration>
            <executions>
              <execution>
                <id>integration-test</id>
                <goals>
                  <goal>integration-test</goal>
                  <goal>verify</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
        </plugins>
      </build>
  </profile>
```