cx playground: add json api endpoint

[SkycoinSynth]

• end- point must take JSON (request id, integer random or string) -- end point accepts a "program"

And returns

• echos the request id back (the string or integer is returned back to sender identically)
• request id string cannot be longer than 256 characters or error

Request for json endpoint returns

• returns program that was input ("program")
• returns execution time in seconds, to 3 decimal places (to ms accuracy)
• has error field, where an error string can be returned (empty string if success). If compiler error occurs, we will use this.
• has "output field" which is string with program output

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Document the list of APIs in readme.md

• with example url
• with example using wget and example output

[Liberxue]

@SkycoinSynth

Visit the playground for the first time to determine whether there is a cookie or uuid (without regenerating a return)

How about all subsequent requests carrying cookies or uuid to confirm user identity?