Merge pull request #154 from geertweening/develop

Memo field support
[TASK] bump ripple-lib to 0.8.2
[DOC] update release notes
[TASK] bump version to 1.3.0-rc2
Fix unnecessary requests in /balances
[FIX] check for issuer for non-xrp payments
Merge pull request #156 from nerdylocks/payments-issuer

[FIX] check and validate issuer upon payment
[FIX] respond with 404 for actNotFound
[TASK] respond with 500's for failed requests
[FIX] use 500 for api errors

402 for requestFailed has been phased out
[TASK] introduce DatabaseError

for communicating errors on the database level
[FEATURE] display version on startup