pcap.py should support nanosecond-resolution pcap files

There is code in pcap.py to support the magic number of both microsecond-resolution and nonsecond-resolution pcap files (if values[0] == 0xA1B2C3D4 or values[0] == 0xA1B23C4D), but the code that resolves the timestamp in a packet header assumes microsecond resolution:

@property
def ts (self):
    """Packet timestamp as a float, a combination of ts_sec and ts_usec"""
    return float(self.ts_sec) + (float(self.ts_usec) / 1e6)

I think if a nanosecond-resolution global header is found (A1B23C4C or byte-reversed) then the timestamp should be ts_sec + ts_usec/1e9. (And perhaps ts_usec is misnamed. Perhaps ts_frac?)

Start in pcap.py by reading the global-header magic-number handling and the Packet.ts property. Verify how microsecond and nanosecond pcap resolutions are identified, then ensure timestamps use the corresponding precision; done when both resolutions produce correct timestamps and the existing pcap coverage includes each case.