[FEATURE] set url_base from config

will override `protocol://host_name:port` when creating transaction status and notification urls
Merge pull request #236 from geertweening/feature/url_base_override

[FEATURE] set url_base from config
[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)