Add currency_pairs api

[nakamegu]

http://techbureau-api-document.readthedocs.io/ja/latest/public/2_individual/2_currency_pairs.html

Example: get data of "btc_jpy"

get_currency_pairs("btc","jpy")
get_currency_pairs("btc")

return

[
    {
        "name": "BTC/JPY",
        "title": "BTC/JPY",
        "currency_pair": "btc_jpy",
        "description": "\u30d3\u30c3\u30c8\u30b3\u30a4\u30f3\u30fb\u65e5\u672c\u5186\u306e\u53d6\u5f15\u3092\u884c\u3046\u3053\u3068\u304c\u3067\u304d\u307e\u3059",
        "is_token": false,
        "event_number": 0,
        "item_unit_min": 0.0001,
        "item_unit_step": 0.0001,
        "aux_unit_min": 5.0,
        "aux_unit_step": 5.0,
        "seq": 0,
        "aux_japanese": "\u65e5\u672c\u5186",
        "item_japanese": "\u30d3\u30c3\u30c8\u30b3\u30a4\u30f3",
        "aux_unit_point": 0
    }
]

Example: get all pairs

get_currency_pairs("all")