[BUG] Empty string is not a valid UINT32
Merge pull request #231 from lumberj/bug/empty_string_not_uint32

[BUG] Empty string is not a valid UINT32
[FEATURE] Balances|Trustlines: Return ledger index (RLJS-127)
[FEATURE] Balances|Trustlines: Use last validated ledger by default
[FEATURE] Balances|Trustlines: Return ledger validated in response

Returns validated (boolean) if the ledger_index returned is validated or
not
[TASK] Balances|Trustlines: Check limit and ledger are valid uint32 and not empty string