Use "asset_issuer" and "asset_code" for payment and trust

[MikeFair]

The parameter names across different operations for custom assets is somewhat non-standard.

In send-payments they are called "token" and "issuer"
In change-trust they are called "code" and "issuer_address"
The allow-trust, path-payment, and manage-offer operations aren't exposed yet

[tolitius]

both send-payment and change-trust now accept "asset_issuer" and "asset_code":

$ bb change-trust -s '{"source_account": "'$(cat foo)'",
                       "asset_code": "TIL",
                       "asset_issuer": "test_issuer*dotkam.com"}'

$ bb send-payment -s '{"from": "'$(cat tmp/distributor)'",
                       "to": "'$(cat foo.pub)'",
                       "amount": "42",
                       "asset_code": "TIL",
                       "asset_issuer": "test_issuer*dotkam.com"}'

the other options are still supported for backward compatibility. will phase them out in later releases