[TASK] remove clone in favor of copy and update

see 'object' case in `parse_json`
[TASK] consolidate accountRequest and pagingAccountRequest

update `requestAccountCurrencies` and  `requestAccountInfo`
[FEATURE] check for valid ledger when using marker

when using a marker on an account request, a valid ledger_index or ledger_hash is required, otherwise the results can't be guaranteed to be reliable

consolidated test values for addresses, indexes and hashes
Merge pull request #203 from geertweening/feature/marker_valid_ledger

[FEATURE] check for valid ledger when using marker