[FEATURE] Balances|Trustlines|Orders: Return ledger index (RLJS-127)

[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

[FEATURE] Orders: Return ledger_index, default to last validated

[TEST] Orders|Balances|Trustlines: Refactor some of the tests

[BUG] Balances: Return XRP balance based on last validated ledger

Also address some comments on PR

[TASK] Trustlines: Remove extra tests, PR comments addressed

[TASK] Orders: Removed unneccesary tests, responding to PR comments
Merge pull request #230 from lumberj/feature/return_ledger_index

[FEATURE] Balances|Trustlines|Orders: Return ledger_index, use last validated ledger by default (RLJS-127)