make the `new` operator optional before the `JSZip` constructor

This line of code implements John Resig's self-calling constructor pattern.

See also:

http://ejohn.org/blog/simple-class-instantiation/

http://habrahabr.ru/post/135027/
+1 test: Constructor adds `new` before itself where necessary
use explicit `instanceof JSZip` check (twice)
Merge pull request #93 from Mithgol/patch-1

make the `new` operator optional before the `JSZip` constructor