[FEATURE] listtrustlines endpoint from account to cold wallet.
[FIX] use "state" to list failed payments

[TEST] All failed payments should have "failed" state.
Merge pull request #121 from ripple/fix-listfailed

Fix listfailed
Merge pull request #119 from ripple/trustlines

Trustlines
[DOC] Add API.md listing all routes.
[DOC] Add subheaders for each api call.
[DOC] Began adding subheaders for api calls.
[DOC] Finish adding method, url as subheaders.
[DOC] Add first few json request, responses.
[DOC] Format API doc.
[DOC] Add more api endpoint json responses.
[DOC] Add short description for each api call.
[DOC] Add API docs to README.
[DOC] Add more api json.
[CHORE] Make user_auth part of webapp config.
Merge pull request #125 from ripple/api-doc

Api doc
[CHORE] Add release notes, bump minor version.
Merge pull request #129 from ripple/release-3.21.0-rc1

Release 3.21.0 rc1
Merge branch 'develop' into release-3.21.0-rc1
Merge pull request #130 from ripple/release-3.21.0-rc1

Release 3.21.0 rc1