Remove tags from Indicator

[brunosan]

I don't know if we really need to have the tags format, available and meta on the Indicators file.

the idea was to specify if the data is "queriable" via API, or a pdf, xls file. On the cases that is API accesible, it would allows to query a particular countriy or year, which is good.

But we can ge tthat information just by looking a the url of the data field. E.g. we know this is queriable from World Bank:

"goal": 1,
      "indicator": " Proportion of population below $1.25 (PPP) per day",
      "data": {
        "url": "http://data.worldbank.org/indicator/SI.POV.DDAY",
        "format": "WB-API",
        "meta": ""
      },
      "source": "http://unsdsn.org/wp-content/uploads/2014/02/140214-SDSN-indicator-report-DRAFT-for-consultation.pdf",
      "leads": "World Bank",
      "other goals": "8",
      "available": "",
      "category": "MDG Indicator, 100"
    }