Added support for testnet, enable when running configure.

To build;
autoreconf -i
./configure --enable-leveldb --enable-testnet
make
sudo make install
sudo ldconfig

Uses testnet3 genesis block and difficulty rules when enabled.

Takes a long time to find initial peers, but eventually starts to download blocks when testing with ./fullnode in the examples. Past block 40000, but still waiting to verify to the full height as I have a slow connection.