Align and rationalize Build profiles

... and fix profile builds (we have several like for example the fact that CLIRR is not executed when the "ci" profile is specified).

Profiles:

• release: generate javadoc, source jars + check java version is 6, that javadoc exist and sing artifacts. Used by release plugin
• clover: skip checkstyle, clirr and enforce checks when running clover
• m2e: set the output dir to target-eclipse for m2eclipse plugin in Eclipse
• legacy: build legacy modules

Properties:

• xwiki.checkstyle.skip: whether or not checkstyle is skipped. False by default
• xwiki.clirr.skip: whether or not clirr is skipped. False by default
• xwiki.enforcer.skip: whether or not enforce checks are skipped. False by default

Checks done:

• enforcer checks
• checkstyle
• license headers

Enforcer checks:

• verify that all plugins are versions specified
• verify the correct junit artifact is used (junit-dep and not junit)
• verify we don't use commons logging or log4j (since we use slf4j)
• verify that the java version is 6 (in the release profile)
• verify that javadoc exist (in the release profile, this is a Maven Central requirement)
• specific check to commons: verify commons modules don't have a dependency on rendering and platform

To execute on CI machine: mvn clean deploy -Plegacy