v0.12.0 getCredentials

Adds a new getCredentials method that can be used to retrieve credentials, either from the previously user provided key, or from the metadata service.

Changes

• 8eddbc9 feat: add getCredentials method (#180)
• b39c16f chore: move from yarn to npm@5 (#172)
• e4d0f5b chore: update gtoken to 1.2.3 (#175)
• 0a810e4 fix: checkIsGCE: report unexpected errors (#164)