Fixed hangs and thread memory leak in Linux

To fix issue: http://www.yafaray.org/node/318

In Linux, when using multiple threads, the missing pthread_join line caused memory leak and eventually filled up the thread stack, causing EAGAIN errors in newly created threads and therefore a complete hang of Yafaray and Blender after rendering several frames.
Fix memory leak in Photon Mapping
To avoid crashes, specially with Photon Mapping
Merge pull request #3 from DavidBluecame/master

Fixed hangs and thread memory leak in Linux