Part of effort to make agoclient.cpp slimmer

This was broken quite a while back (9aafd93e)

Also update test_agorpc.py a bit

Support for jsoncpp differs wildly between distributions. jsoncpp is
small and has support for "amalgamated" version to be included in
applications.

mongoose code and supporting code have been moved to new
shared/agohttp/ library which agorpc utilizes.

example: humidity change with "unit":"%"

If data is missing, omit it from jsonrpc response.
If data is not a map, return error instead of crashing.
Use different Jsonrpc error codes for malformed message and actual remote error

["type"] will create a blank element if it did not exists before. This breaks some UI code which does not expect any keys with null values

As a mg connection may be reused (connection keep-alive), we must free
the GetEventState object every time we're done with the request, not only when
connection is closed.
Since we now clear it, we don't need to keep track of "processed" state.

Fix lost event in agorpc: when 2 events are too close, agorpc calls getEventRequest in eventHandler and almost at the same time in MG_POLL. First event is writen in event response but second one is lost. Fix was to set request has already been processed.

C++ whitespace & indentation cleanups in shared + core, as per decision in #135 and related discussion