Issue99: allow to cleanup threadlocals. check existence of closure in cu...

...rrent thread local when binding closures

Hi,
I have created fix to Issue 99 I have reported. There are new methods to clean up thread locals to prevent memory leak while running at thread-pooled environment with custom class loaders - for example: running on Tomcat's WebappClassLoader.
Problematic is reference to web app classloader while application reload. 
ThreadPool -> Thread -> ThreadLocal -> INSTANCE <- Class <- ClassLoader .
I was thinking about using Weak/SoftReference but they do not fit enough to this problem.
I hope this fix is good enough :]
Ivos