Rebuild on pom change

After a succesful build, all classes are compiled.
Following builds won't recompile the classes since the sources
haven't changed.

When a user changes a dependency in the pom and then mvn installs, the sources
aren't checked with the modified dependency.

I propose we save a timestamp (or the pom itself) in target/ for each build,
and let plugins that do some sort intelligent checking take this into account.

There are a few options:

• define a default location for the pom and let plugins implement timestamp or content checking
  as they please
• define some api to ask if the project has changed
• bind the clean plugin to some startup phase and let it auto-clean if the pom (or possibly parent poms/dependent poms)
  have changed.