Fix deadlock issue in #50

[elbakramer]

This addresses issue #50

Main idea is from @fxgendrin in the issue #50

Need a new release with following modification in AbstractClassLoader (as suggested in comment, but need to be compatible only with java 1.6, alternative is to duplicate ConcurrentSkipListSet into the project)
protected final Set<ProxyClassLoader> loaders = new ConcurrentSkipListSet<ProxyClassLoader>();

Besides the original proposal, there are some minor tweaks

• Used backport implementation of java.util.concurrent in order to support java 1.5+, while some of the required classes are available since 1.6
• To prevent loaders becoming Set from List, introduced thin wrapper(SetList) that wraps Set as List
• Removed synchronized related codes/blocks because now java.util.concurrent handles the concurrent access/modification.

Checked that all existing tests are passing. But cannot think of a way to test properly that this really resolves the problem at hand.

Actually, my first choice was to ignore java 1.5+ support and just to use java.util.concurrent package. I tried that in my use case and it seemed to work as I expected.

[sandrocsimas]

The same problem here. It seems that the DEV is not active on this project anymore =(