Added download-all feature

The button to download all the files in a thread only appears when the
page is a thread, such as treeview. I added the download-all button next
to the treeview one. I modified stylesheets/v8ch.css to include the id
#download-all. I modified inc/instance-config.php to include the
download-all.js and jszip.min.js. I added the files js/jszip.min.js
which is a library to work with zip files in JS and I added
js/download-all.js which goes through all files, adds them to a zip and
then gives it to the user.
Revert "Added download-all feature"

This reverts commit 12a8f1a672f8c6ce8f46b6267e5d836d379d9514.
Added download-all feature without instanceconfig

The button to download all the files in a thread only appears when the
page is a thread, such as treeview.
I added the download-all button next
to the treeview one.
I modified stylesheets/v8ch.css to include the id
#download-all.
I DIDN'T modified inc/instance-config.php to include the
download-all.js and jszip.min.js. That goes in the next commit.

I added the files js/jszip.min.js
which is a library to work with zip files in JS and I added
js/download-all.js which goes through all files, adds them to a zip and
then gives it to the user.
Modified inc/instance-config.php

Modified inc/instance-config.php to include js/download-all.js and
js/jszip.min.js
Removed 8chan reference in js/download-all.js
Changed x from being a local var to a global
Merge pull request #155 from hugofragata/master

Added download-all feature