Issue title: fix css include
 Issue body: Hello. Please fix your bower dependencies, like
"main": ["dist/cropper.js", "dist/cropper.css"]
becouse wiredep isn't including css file.

`"dist/cropper.css"` - this line

 Comments: 
 Comment 0: OK~ Thanks~
{'source': 'GithubIssues', 'repo_name_with_owner': 'fengyuanchen/cropper', 'issue_id': 56634605, 'repo_stars': 7802.0}