Walker and SnapRemover improvements.

Changes:
* walker made able to perform "breadth-first" walks
* snap remover made able to reuse items from walker

Conflicts:
	components/nexus-core/src/test/java/org/sonatype/nexus/proxy/walker/WalkerTest.java
Removed printlns and added Walker test for breadth-first
Merge pull request #454 from sonatype/nexus-2.8.x-snap-remover-enhancements

[2.8.x] Walker and SnapRemover improvements.