add thor class for standalone reading

[libotony]

This PR is on top of #148, adds Connex.Thor class for the below usage:

const thor = new Connex.Thor({node: 'https://...', network: 'main'})
// .... some reading options
const vendor = new Connex.Vendor(...)
vendor.sign('tx', ...)

In some cases of dApps, they need to show data before asking the user to choose a wallet.