Support sending gzipped request bodies

[alahiff]

Whenever there is a large amount of content the Python API should gzip the body and include the following header:

"Content-Encoding": "gzip"

This should be done for:

• Sending large numbers of runs, i.e. "loadSim"
• Sending scalar or non-scalar metrics
• Sending events

In the requests post you can just do:

data=gzip.compress(msgpack.packb(data, use_bin_type=True))

or

data=gzip.compress(json.dumps(data).encode("utf-8"))

as appropriate. Obviously don't use json=... any more in requests.post(...), use data=... instead.

[alahiff]

Should only do this if the server version is >= 1.3.0.