natvis and pretty printers

[vinniefalco]

We need natvis and gdb pretty printer support. I think the natvis is already there. We need the gdb pretty printer. Furthermore we need tests. Each test would visualize a predefined object (say, a local variable in a main function) and then compare the output to make sure it exactly matches a string constant:

you write a main that declares a local json::value with some stuff in it and then using a shell script you drive gdb to get a breakpoint and visualize the variable to cout. Then you compare the result with a string literal