Add 'Main-Class' header to slf4j-migrator Manifest.MF

[HannesWell]

Because the Manifest.MF did not have a terminating new-line it was invalid and its content was ignored.

Disable the execution of the 'maven-bundle-plugin' for the migrator module to not have (useless) OSGi-metadata generated into it.

[HannesWell]

@ceki maybe do you want to have a look at this as well. Because of this the migration can probably not used as described here:
https://www.slf4j.org/migrator.html

Alternatively this entry could be generated using the maven-jar-plugin, but since the intention is to use the maven-bundle-plugin in all other plugins to generate Manifest-entries (#340) I choose to use it here as well.

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
          <archive>
            <manifest>
              <mainClass>org.slf4j.migrator.Main</mainClass>
            </manifest>
          </archive>
        </configuration>
      </plugin>