Complete the web API for the wallet vs. the server

[itamarro]

Should include calls for:

1. Getting the payment zone details from a webpage (including the Bitcoin address to deposit Bitcoin to)
2. Perform a payment on the Cosmos chain
3. Get the user's current Bitcoin balance on the Cosmos chain

[itamarro]

For the MVP here's my suggestion.

Given that there can be any number of payment zones (anyone can start a payment zone - that's the point), when interacting with a website that receives payments in a payment zone (PZ), the website needs to tell you what PZ to use.

The browser wallet plugin will get the payment zone server address from a 'meta' field in the loaded page:

<meta property="pz:server" content="http://www.example.com/pz">

The plugin thus knows what server it needs to interact with. That is the server it will access for the PZ interactions for that same page.

(The wallet client-side API will allow to the page to request a payment in the PZ, specifying destination address and amount. The plugin will show a user an approval dialog when that happens.)

The payment zone server will offer three methods:

1. Getting the Bitcoin address to make deposits into this PZ
2. Getting the balance of a Bitcoin address in this particular PZ cosmos chain
3. Making a payment from one Bitcoin address to another on this particular PZ

HTTP-wise it'll look as follows:

GET /pz-wallet-address
GET /balance/<BITCOIN-ADDRESS>
POST /pay/<FROM-BITCOIN-ADDRESS>/<TO-BITCOIN-ADDRESS>/<AMOUNT-IN-SATOSHIS>

The first 2 are quite straightforward.
The "pay" call payload needs to include some proof of ownership, otherwise anyone can make a payment in the name of anyone. Details of that would be fleshed out later, see #3.

[itamarro]

I've committed the stubs for this API: CosmicBridge/server@37fb8c5

Next step - integrate with Cosmos.