Release v3.1.0

[dduponchel]

Changes from v3.0.0:

• utils.delay: use macro tasks instead of micro tasks (see utils.delay: use macro tasks instead of micro tasks. #288).
• Harden base64 decode (see Harden base64 decode. #316).
• Add JSZip.version and the version in the header (see Add JSZip.version and the version in the header. #317).
• Support Promise(Blob) (see Support Promise[Blob]. #318).
• Change JSZip.external.Promise implementation (see Change JSZip.external.Promise implementation. #321).
• Update pako to v1.0.2 to fix a DEFLATE bug (see Update pako dependency version to 1.0.2. #322).