Updated README.md.

Summarized the introduction and added a brief usage guide.
Added further features to and debugged ajax barcode scanner.
1. debugged call to scanSuccessCallback after scanstop is activated
2. added public method / API for start/stop scan
3. added cookie to remember videoChoice
Added and debugged barcode_scanner.
1. debugged call to scanSuccessCallback when scanoff is activated
2. added public method / API for startScan & stopScan
3. added cookie memory of videoChoice
Merge branch 'master' of github.com:gabrielwu84/zbar-web-interface