[FEATURE] Add direction to ripple transactions.

[TEST] Ripple transaction direction validates in set.
[FEATURE] Associate ripple addresses and ripple transactions.

[FEATURE] Return associated adresses in /v1/ripple_transactions.

[FEATURE] Begin testing associations implementation.

[FOLD] Spacing.

[TEST] Create and tear down data, not hard-code.
[FEATURE] Add invoice_id to external transactions.
Merge branch 'develop' of github.com:ripple/gatewayd into develop
[FEATURE] submit memos & invoice_id to the ledger

[FEATURE] add sha-256 validator

[TEST] submit/validate memos & invoice_id