image upload: added strict mode

Using:
add attr source-strict="true"
image upload: added check for min size

Using:
add attr check-min-size="true"
image upload: better alert message
Merge pull request #33 from rokerkony/image-upload-update

Image upload update, strict mode & check for minimum dimensions
Merge branch 'master' of github.com:lmc-eu/ngx-library