Add is_mine property to listtokens rpc method

[sakdeniz]

Currently listtokens method returns all available tokens in the blockchain.

I think it is necessary to add the is_mine attribute to every token object so that we can understand which of these tokens belongs to the wallet owner.

Sample :

  {
    "version": 0,
    "is_mine":true,
    "id": "777896868f6b80710b42923a0ad51bf6ff73fb0cbd727ad2d617c4c338683fca",
    "pubkey": "99259540dc2fae3fd6c3205be105f202c8416fe87433833f810ac255c422d26a78c4d3e594949f0ec17e820265b15f9a",
    "name": "Baby NAV",
    "token_code": "BNAV",
    "current_supply": "200.00",
    "max_supply": "100000000.00",
    "balance": "0.00"
  },

[mxaddict]

I can work on this unless @aguycalled is already working on it.