[FIX] Validate issuer ripple addresses, allow null.

[TASK] Bump patch version.

[FIX] Override to_issuer, from_issuer setters, test.
Merge pull request #436 from ripple/validate-issuers

Validate issuers
[FEATURE] Extract user routes into express.Router.
[TASK] Migrate apiRouter to express.Router.
[TASK] Remove all behavior from lib/ripple.