Change capability for EDD Add Ons and System Info sub menus

The "install_plugins" capability gives only WP super admins on WP multisite installs access to Add Ons, System Info Upgrade screen.
This is how it should be.
On single WP installs the Admin still can access these sub menus.
see
http://codex.wordpress.org/Roles_and_Capabilities
Updated readme with campaign tracking links
Merge pull request #1296 from fxbenard/master

[i18n]> french and pot updates
Correct test mode check in edd_complete_purchase
Merge branch 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads
Add data-download-id to cart rows and make sure ID rows are unique. CLoses #1284
Change Download to Free Download. CLoses #1253
Correct wrong version number in readme
Fix incorrect redirect when selecting a payment method with ajax disabled and permalinks set to default. Closes #1290
Hide discount field is cart total is 0.00. Closes #1286
Check for status of publish, not complete. Closes #1277
Sanitize email subject with wp_strip_all_tags(). Closes #1283
Update param name to  and add to edd_Cart_item_price filter. Closes #1289
Clean up indention for edd_get_ip()
Fixed undefined index . #1270
remove unused
Clean up edd_get_file_ctype() formatting
Clean up edd_get_file_ctype() formatting
fixes duplicate array key in add_roles
fixes duplicate array key in currencies