add javadoc to metrics-lib

Steps for providing javadoc for metrics-lib:

• Change method comments to javadoc style, i.e. from /* ... */ to /** ... */

• Add a javadoc ant-task that renders the documentation into a seperate folder docs.

• Make sure the contents of the comments is still up-to-date.

These steps should be provided in three patches. Test classes should not be included yet.