Add dynamic loading loading mechanism for CommitFilters.

This PR uses the [SPI mechanism provided by the JRE](http://docs.oracle.com/javase/6/docs/api/java/util/ServiceLoader.html) to load all implementing classes of the CommitFilter interface. This way it is possible to add a dependency to the plugin configuration in order add new filters dynamically. For example, using the following configuration:
```
<plugin>
  <groupId>com.github.danielflower.mavenplugins</groupId>
  <artifactId>maven-gitlog-plugin</artifactId>
  <version>1.5.3-SNAPSHOT</version>
  <configuration>
    <generateSimpleHTMLChangeLog>false</generateSimpleHTMLChangeLog>
    <generatePlainTextChangeLog>true</generatePlainTextChangeLog>
  </configuration>
  <executions>
    <execution>
      <phase>package</phase>
      <goals>
        <goal>generate</goal>
      </goals>
    </execution>
  </executions>
  <dependencies>
    <dependency>
      <groupId>es.e-ucm.ead</groupId>
      <artifactId>gitlog-maven-plugin-ext</artifactId>
      <version>0.0.1-SNAPSHOT</version>
    </dependency>
  </dependencies>
</plugin>
```
loads the CommitFilters of the es.e-ucm.ead:gitlog-maven-plugin-ext maven artifact.