Add JSON API to imageflow_server

[lilith]

I'd like to wait until I get feedback on the imageflow_tool JSON API before I implement it in imageflow_server.

[gerneio]

@lilith just wanted to check to see when you think this would be implemented? I have no desire to write those complicated-looking JSON request objects and would much rather use a fluent API, but for features that do not yet have a Fluent API, this at least gives us an entry point. So for example, I noticed that FillRectangle doesn't seem to be exposed in imageflow-dotnet-server, but is available in imageflow-dotnet. So my guess is that there are still quite a few methods/properties that aren't exposed in the Fluent API on imageflow-dotnet-server yet.

[lilith]

imageflow_server is obsolete: #598

[lilith]

Moved to imazen/imageflow-server#59