Document enums and flags (and make them simpler to use?)

[Mr0grog]

This basically the same issue as lightsail-network/java-stellar-sdk#31, but for the JS SDK.

It's possible to use enums from the generated XDR classes to make code clearer, as in setting flags on an account:

StellarSdk.Operation.setOptions({
  setFlags: StellarSdk.xdr.AccountFlags.authRevocableFlag().value |
    StellarSdk.xdr.AccountFlags.authRequiredFlag().value
});

// or slightly less awkward if you make yourself a shortcut:
var flags = StellarSdk.xdr.AccountFlags;
StellarSdk.Operation.setOptions({
  setFlags: flags.authRevocableFlag().value | flags.authRequiredFlag().value
});

It'd be helpful if these were documented.

It might also be nice if working with these was easier, though that's maybe better handled as a separate issue? (maybe the setFlags option could take an object, or the flags were exposed at the root level, or the flags weren't methods and you didn't have to get the value property from them, etc.).