GStreamer data protocol

Intended to wrap GstData objects in a line protocol for use with
pipe/network elements.

IDEAS
-----
* for transporting buffers, have a function that creates a header for
  a given buffer to be written before the buffer.  This way, you don't lose
  time creating a GDP buffer from the buffer
* allow for CRC'ing of the GstBuffer, optionally
* have a version number of the protocol
* optimizing the header for size is not useful since the GstData structure
  already contains more than 32 bytes anyway, making up half the header size

PROTOCOL
--------
* 1 byte   GDP major version
* 1 byte   GDP minor version
* 1 byte   GDP flags
* 1 byte   GDP data type
           1 for buffer, 2 caps, 65+ for events
* 4 byte   length of "payload"
* 8 byte   timestamp of buffer/event
* 8 byte   duration  of buffer
* 8 byte   offset
* 8 byte   offset_end
* 16 byte  future extension (ABI padding)
* 2 byte   optional header crc
* 2 byte   optional payload crc
* (length) payload

header: 60 bytes

all numbers use network byte ordering.