feat(api): add category on uiviewcontroller

the category can be used to open the scanner for a single scan. it has a completion handler that is called once the first code is detected on screen.
chore(project): update umbrella header
feat(example): add a button that makes use of the uiviewcontroller category
Merge pull request #8 from michaelochs/feature/#6-block_based_api

block based api