Review big numbers in javascript code.

We need to review any numbers that are potentially huge (e.g. transaction amounts, utxo positions) and use BigNumber.js where necessary.

For example:
`omg-wallet.js` line 197 should be:
 ```
const selected = correctCurrency.find(utxo => new BigNumber(utxo.amount).gte(new BigNumber(amount)))
```

And line 161:
```
if (new BigNumber(utxosToSpend[0].amount).gt(new BigNumber(value))) {
```
