Add JSON API for submissions

[kmyk]

I think the below is a example of the minimal interface.

{
  "url": "https://atcoder.jp/contests/abc160/submissions/11991846",
  "problem": {
    "url": "https://atcoder.jp/contests/abc160/tasks/abc160_a",
    "name": "Coffee"
  },
  "verdict": "AC",
  "code": "#!/usr/bin/env python3_, _, c, d, e, f = input()\nprint(c == d and e == f and 'Yes' or 'No')\n"
}

memo:

• When we add the field for users, we should make a user object like "user": { "url": "https://atcoder.jp/users/kimiyuki", "name": "kimiyuki" }
• When we add the field for language, we should use the language object like "language": { "id": null, "description": "Python3 (3.4.3)" }
• If we add a field for memory usage, I think we should use MB for memory usage to keep the compatibility with get-problem, which also maintain the compatibility with competitive-companion. Or, we should use bytes for all fields and break the compatibility with competitive-companion.
• Some people want this API to show verdicts in real-time.