Add support for Fine-Grained Control of Authorization

[fracek]

CAP 18 changed the data type for the attribute authorize in AllowTrustOp. It is now a uint32 instead of a boolean. It also includes a new TrustLineFlag which is authorizedToMaintainLiabilitiesFlag with value 2.

You can see the implementation in js-stellar-base here, which still accepts booleans but converts the value to an uint32.

This is part of #246

[Kirbyrawr]

Since we added the authorizedToMaintainLiabilities in other places i will follow the Java's code instead as i think is more easy to read.
https://github.com/stellar/java-stellar-sdk/pull/276/files#diff-dc1e56016276f11d48a7766517a79e9bR69

[Kirbyrawr]

I believe it's only these changes
#250

[fracek]

Fixed by #250