Save image into db, security (see description)

- Adding methods to working with image (save, get, write, delete) in
MongoDB. They added in ProductService.
- HomeController has a small demo to save, then get & write an image
from specific path.
- Some config for Spring security, but useless =.=" Uncomment in file
'web.xml' (line 38-46) for a small demo. Username & password define in
file 'security-app-context.xml'.