Add WebKitDownload:allow-overwrite object property

Closes #17
Bind webkit_web_view_load_bytes

Closes #25
run-travis: get libwebkit2gtk-4-0 from vivid/universe
Add webkit2.user-content-manager.lisp

Only exposes the user content manager type itself.
Bind webkit_web_context_allow_tls_certificate_for_host

Closes #24