[api] Explorer URL

[GET] /api/addresses/{address}/explorer-url

Should return one or many blockchain explorer URLs for the given address.

Response:

```js
[
// Blockchain explorer URL for the given address “string”
]
```

Python implementation

```py

@api.route('/addresses//explorer-url', methods=['GET'])
def get_explorer_url(address):
"""
Returns explorer url for given address
"""
if address in (None, ''):
return make_response(jsonify(build_error('Address not specified')), 400)
elif not isValidAddress(address):
return make_response(jsonify(build_error('Invalid address')), 204)

explorer_urls = ['https://explorer.skycoin.net/app/address/' + address]
return jsonify(explorer_urls)
```