Start writing some docs about RPC protocol.

This is only the beginning: unfortunately I just noticed a nasty
bug which I prefer to take of immediately.
Async: remove some unused code.
Async: remove length at the beginning of RPC messages.

This fix a nasty bug that would have been triggered by having
a JSON length that contains the bytes '\r\n', which are used for
message termination. Fortunately our messages are always short
enough that, to my knowledge, this bug has never been triggered.
Some more documentation about internals.
Comments in cmsMake.
Fix small bug in YamlLoader.
A couple of typos.
Loaders: make short_name and description attributes.
Fix a couple of docstrings.
Fix another bug in YamlLoader.

Now the conversion in load() gets applied also if None was to be
returned in the case of a non existing key.