Fix getBtcBalance wrapper response

[tmalahie]

The wallet.getBtcBalance method returns the data as a string instead of an object. All similar methods do a JSON.parse before to return an object.
This PR resolves the inconsistency by adding the missing parse

[zoedberg]

As we discussed in #2, node.js suffers float rounding errors, therefore we decided to get and return all numbers as strings. Then we suggest that users parse those strings with some specific library, as big.js. Therefore I'm closing this PR

[zoedberg]

Oops, sorry, my mistake. I forgot that the get_btc_balance returns an object and not a single amount. I'm afraid though that the parsing will not convert the numbers to strings. So we should implement something like we did for getting the numbers as strings (i.e. #[serde(deserialize_with = "from_str_or_number_optional")] and #[serde(deserialize_with = "from_str_or_number_mandatory")]). Therefore I'll reopen this PR

[nicbus]

a couple points:

• please format the code
• in example.js, please call JSON.stringify on btcBalance so it gets properly displayed

[zoedberg]

The changes look good, thanks. Please squash the commits into a single signed one, then I'll merge this

[zoedberg]

LGTM, thank you!