[FEATURE] Auto-set LAST_PAYMENT_HASH if not in config.

[FOLD] Start at hash if LAST_PAYMENT_HASH if config.
Merge pull request #335 from ripple/auto-set-last-hash

[FEATURE] Auto-set LAST_PAYMENT_HASH if not in config.
[FIX] add destination tag if there is any
[TEST] assert for sent not successed on _confirmSuccessfulPayment
[TASK] add bluebird npm dependency
Merge pull request #336 from ripple/outgoing-destination-tag

Add destination tag outgoing payment if provided
Merge branch 'develop' of github.com:ripple/gatewayd into develop
[DOC] Add gatewayd-admin guide

For running, using, and developing gatewayd-admin