allow executions with no id

currently, the following doesn't pick up the execution configuration:
<plugin>
<groupId>org.codehaus.modello</groupId>
<artifactId>modello-maven-plugin</artifactId>
<version>1.0-alpha-3-SNAPSHOT</version>
<executions>
<execution>
<configuration>
<version>1.0.0</version>
<packageWithVersion>false</packageWithVersion>
<model>src/main/resources/continuum.mdo</model>
</configuration>
<goals>
<goal>java</goal>
<goal>jpox-jdo-mapping</goal>
<goal>jpox-store</goal>
</goals>
</execution>
</executions>
</plugin>

When an id is added, the configuration is picked up. I assume this slipped through because the original spec called for an id on the execution.

Requiring an id is probably too strict. What we should do is make id optional (as it is), honour it as an execution, and its configuration, but not merge it with any other executions with or without an id (so they would all just be added as new executions in inheritence).