Switched to Berkeley DB 4. Other changes (see details below!!!)

- got rid of gthread
+ removed a bunch of unused and irrelevant-to-osx stuff from Makefile
+ no need to specify `USE_UPNP=1`, it is done for you (make sure to install it!)

Instructions:

1. `brew install openssl berkeley-db4 boost miniupnpc`
2. cd src && make -f Makefile.osx -j $(sysctl -n hw.ncpu) all