[BUG] Orders|Trustlines|Balances: Allow ledger hash query parameter

Allows ledger strings (validated|closed|current|), hashes, and
sequences. Also, adds tests for the additional string parameters.

(RLJS-137)
Merge pull request #235 from lumberj/bug/allow_ledger_hash

[BUG] Orders|Trustlines|Balances: Allow ledger hash query parameter (RLJS-137)