x/token: Fix to update the total supply of all coins when issuing new tokens

[youngjoon-lee]

Fixes #93

No unit test yet. Unit tests will be added soon by following the Cosmos-SDK's way (probably in the newer Cosmos version).

A manual test was done:

panacead export --for-zero-height --height=10

    "supply": {
      "supply": [
        {
          "amount": "1000000000",
          "denom": "ulov221"
        },
        {
          "amount": "100000001584400",
          "denom": "umed"
        }
      ]
    },
    "token": {
      "tokens": {
        "LOV-221": {
          "mintable": false,
          "name": "love",
          "owner_address": "panacea1xs5e3p3jxfxr3n9xluagkgnxxk9vpcm0dcrew2",
          "symbol": "LOV-221",
          "total_supply": {
            "amount": "1000000000",
            "denom": "ulov221"
          }
        }
      }
    }

[cl9200]

LGTM