Make edd_get_cart_discounts_html() function filterable

Allows discount html code to be filtered + fix spelling
Working sidebar / multi insert. Props Chris Kelley #612
Changed $discount to $discounts
Merge pull request #841 from sumobi/checkout

Make edd_get_cart_discounts_html() function filterable
Fix dashboard breakage
Merge pull request #853 from ghost1227/master

Dashboard widget breaks dashboard
Serialize form for AJAX request

Serialize the form instead of posting only EDD inputs. Allows for posting of custom fields via AJAX.
Fix syntax error

Replace comma with ;
Introduced edd_get_lowest_price_option(). #727
Introduced edd_get_highest_price_option(). #727
Introduced edd_price_range(). #727
Show price range in download wp list table. #727
Make sure blank prices aren't taken into consideration when finding range. #727
Reindex repeatable fields when deleting a row. Fixes #823
Alert user if checkout page doesn't exist. Fixes #754
Unset default media tabs. #612
Updated readme
Localize EDD_VERSION
Make it possible to disable new media UI (Amazon S3 extension requires it). #612
Merge pull request #858 from pippinsplugins/media-ui-dev

Media ui dev