Add listSubscriptions method to API

There are currently `addSubscription` and `deleteSubscription` methods for the API, but there is no way to iterate over existing subscriptions.  This pull request adds the api method listSubscriptions, which returns subscriptions in the following json format:

    {"subscriptions":[{
    "address": "BM-GtovgYdgs7qXPkoYaRgrLFuFKz1SFpsw",
    "enabled": true,
    "label": "Qml0bWVzc2FnZSBuZXcgcmVsZWFzZXMvYW5ub3VuY2VtZW50cw==\n"
    },{...}, ...]}