documentation : fix link
documentation : clarify the wording of ZipObject.

Saying that "the content may need to be decompressed/converted first"
may confuse users, see #132.
documentation : display a warning in IE 6-9

See #125. Also, don't imply that the example contains the flash polyfill.
documentation : explain how to get the JSZip object.

Fix #118.
Merge pull request #133 from dduponchel/update_docs

Update documentation