feat(uploader.api.js): `_showDeleteConfirm` can deal with (non)-promises

References: #719
refactor(uploader.js): `showConfirm` is simplified

References: #719
refactor(uploader.js): `showPrompt` is simplified

References: #719
feat(uploader.api.js): `_showDeleteConfirm` can deal with (non)-promises

References: #719
refactor(uploader.js): `showConfirm` is simplified

References: #719
refactor(uploader.js): `showPrompt` is simplified

References: #719
refactor(uploader.js): Use else-if for promissory return val check

Simplified the logic a bit.

references: #719
chore(uploader.api.js): Remove extra comment
Merge branch 'feature/promissory-show-functions' of github.com:Widen/fine-uploader into feature/promissory-show-functions

Conflicts:
	client/js/uploader.api.js
	client/js/uploader.js
Merge branch 'feature/promissory-show-functions' into develop

closes #719
docs(features/promises, styling): Document promissory `show` methods

Recently, `showConfirm` was made promissory, so I documented it.