The test added here is copying the first few steps of the previous test,
"alice swaps bob 10 A for 1 B". For some reason, the second time you
simulate this transaction, it is calculating that no non-invoker signing
is needed! Haven't figured out why yet.

Note that this is an order-dependent test. The new one only functions as
expected because mocha runs the tests in the same order every time.

Adds a toXDR and fromXDR for AssembledTransaction class. 
Also adds a txFromXDR to the Client class. 
Changes the swap test to use this flow.

Limitations: If you use the XDR multi-auth flow, you must re-simulate 
before the final `signAndSend` call. This is due to the fact that we can't 
serialize the XDR of the transaction envelope with the results of the 
simulation. If we want to do this, we would need to add an 
`AssembledTransaction` type to XDR in `stellar-base`.
---------

Co-authored-by: George <[email protected]>

The new test added here shows that you can edit a transaction with
`tx.raw = cloneFrom(tx.build)` and that the `tx.simulationData` will be
updated correctly.

Co-authored-by: Chad Ostrowski <[email protected]>
Co-authored-by: blaineheffron <[email protected]>
Co-authored-by: George <[email protected]>

…ion` (stellar#992)

* add auto restore functionality for contract client
* extract getAccount to utils
* add changelog entry for auto restore functionality

Co-authored-by: Chad Ostrowski <[email protected]>
Co-authored-by: blaineheffron <[email protected]>

* Upgrade dev dependencies
* Upgrade stellar-base to latest version