title: How to convert RSK token balance to a Javascript number?
body: <p>I want to get the balance (number of tokens) from the token smart contract.</p>
  <p>I'm using web3.js to interact with the contract, and am able to get a return value.
  However, with this value, if I do <code>.toString()</code>, I see that it has the correct value.
  However, if I do <code>.toNumber()</code>, it gives me an error:
  <code>Error: Number can only safely store up to 53 bits</code></p>
  <p>Why does this happen? And how can I get the balance of a particular account from a smart contract, <strong>as  a number</strong> (not a string)?</p>