Reorganize into mother and child pom

[johnerikhalse]

Webarchive-commons is a quite big library with lots of dependencies on third party libraries. It should be split into smaller parts to enable projects to include only what they need.

The first step in this direction is to split the project into a mother pom and a child pom. It will not alter the library itself (keeping group-id and artifact-id), but will enable us to later on split functionality into sub-libraries or adding new stuff as a separate sub-library.

[johnerikhalse]

After giving the naming a second tought, I think it is better to rename the current artifact to webarchive-commons-core and reserving the current artifact name webarchive-commons for the mother pom. Then this should wait for 1.2 release.

[johnerikhalse]

Even if this is not a big change for other libraries depending on webarchive-commons, it is about moving code around and that might disturb forks which is not in sync with master. Postponing it to 2.0.0 where compatibility might break anyway.