release: upgrade to use local did-resolver (#50)

* change iat to nbf

* feat: Set `iat` timestamp by default instead of `nbf` (#43)

* refactor: change createJWT to set iat by default instead of nbf

* test: add tests for iat behavior in createJWT

* feat: do not set iat if value in payload is null

* fix: set payload.iat to undefined instead of null to omit from JWT

* fix code style for division

* fix code style

BREAKING CHANGE: expiresIn option passed to createJWT now calculates exp from nbf instead of iat.

* chore: add release automation configuration (#44)

* feat: verifyJWT against nbf if present but use iat if not (#45)

* feat: verifyJWT against nbf if present but use iat if not

* fix: apply skew in the opposite direction when checking exp

* test: make some test inputs more clear

* test: use hard coded JWTs to test validFrom teimstamp

* style: remove unused constants

* feat: upgrade to local did resolver (#48)

* feat: upgrade to local did resolver

* refactor: export resolver so it can be mocked in tests

* wip: refactor tests to remove dependency on uport and nacl did resolvers

* test: fix tests to verify pregenerated JWT

* test: fix hard coded jwts for validFrom timestamp tests

* test: fix  remaining

* test: add tests for normalizeDID

* refactor: upgrade ethr-did-resolver to fix imports

* refactor: rename variables and clean up async syntax for tests