separate attribution comment & use "/*!" for it

Use "/*!" to ensure attribution comment is preserved by minifiers
Merge branch 'patch-1' of https://github.com/cvrebert/jszip into cvrebert-patch-1

Conflicts:
	jszip.js
change the grunt config for the issue #80

This commit updates the grunt configuration to ensure that the license
comment (starting with /*! for the minifiers) makes its way to the final
dist files.
Remove the "usage" block from the license
Merge pull request #81 from dduponchel/grunt-issue-80

change the grunt config for the issue #80