rename dirent.h into directory_entry.h to not clash with <dirent.h>
include testees with relative pathes
add testee dependencies to CMakeLists.txt
add TryLock() to Lockable base class
add unit tests for LockGuard<> and Lockable
add tests for basic callback functionality
add a Observable<> test
add test separation marks (code readability)
add unit tests for FifoChannel<> both multi and single threaded
add CMakeLists dependency to util_concurrency_impl.h
Merge branch 'test_concurrency' of git://github.com/reneme/cvmfs into reneme-test_concurrency