Use built-in linking rules (with its own variables).
Refer to the json-c library as per their examples.

Not using -I makes more sense to me, but it appears that some distributions
put the headers in different locations, and all the example code refers only
to json.h, so we'll follow suit...

Also link against the correct library.
Remove some unneeded whitespace.
Add a .md extension to go/README.
Update the content of go/README.md to Markdown.
Convert go/README to Markdown format.
Remove the nearly useless go/ENV file.
Reformat merkle_tree_go.cc.
Add C++ to the continuous build.