Add pcap_inject & allow monitor mode

While one might argue that this library should be solely for packet inspection, it is unquestionable that the authors of libpcap also had packet *injection* in mind when they wrote their library.  Since this repository is such a nice binding for libpcap, it only makes sense to support `pcap_inject()`.  Packets can now be injected via:

    session.inject(buffer);

Furthermore, monitor mode is crucial for many wireless packet scanning operations.  Because of this, I went ahead and solved one of he TODO's in the source code.  Now, a monitor-mode 802.11 pcap session can be created liked this:

    pcap.createSession('en0', '', 65536, true);

I have personally tested both of these new features, and they behave exactly as expected.