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
fix the script src in the index.html
Add support for the unicode path extra field

This patch sets the unicode path extra field. unzip needs at least one
extra field to correctly handle unicode path, so using the path is as
good as any other information. This could improve the situation with
other archive managers too.

This field is usually used without the utf8 flag, with a non unicode
path in the header (winrar, winzip). This helps (a bit) with the messy
Windows' default compressed folders feature but breaks on p7zip which
doesn't seek the unicode path extra field.

So for now, UTF-8 everywhere !

Fix #79.
Merge pull request #82 from dduponchel/unicode_extra_field

Add support for the unicode path extra field
Merge pull request #81 from dduponchel/grunt-issue-80

change the grunt config for the issue #80