Added get_tx_metadata when transaction is not relayed

[euri10]

Fixes #123

[euri10]

question is should I do the same modification to the sweep_all mehtod ?

edit: I did

[euri10]

the changes I made impact the transfer and sweep_all methods, and as the test_sweep_all uses relay=False it's a goog opportunity to get those changes tested, so the json test_sweep_all-20-sweep_all.json need to have a "tx_metadata_list" added.

@emesik I'd need a little guidance on how you generate the json files used in the tests so I can modify it so that test_sweep_all uses passes, it currently fails and righly so

[xmrluvr45]

works for me, this patch allows to check tx fee before using the relay_tx method to relay the transaction metadata hex.