Set userId in MDC on login, current filter impl of MDC handling is not sufficent
Add custom WebSecurityManager impl and override login/logout to set MDC userId attribute.
Drop setting of MDC userId attribute from authc filter.
Split off MDC userId attribute from MDCUtils (nexus-core) into UserIdMdcHelper (nexus-security).
fix isSet negate entire expression
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
process annotations for classes used by xstream for tests

Conflicts:
	components/nexus-core/src/test/java/org/sonatype/nexus/configuration/ClearPasswordTest.java
Merge pull request #302 from sonatype/set-mdc-userid-onlogin

REVIEW: Set userId in MDC on login
Merge remote-tracking branch 'origin/master' into as-expired-fixes