Adding filters to admin script loader

Adds three more filters to the edd_load_admin_scripts() function to
allow extension pages to use the same scripts.
Last comma removed in array
Fix for issue #1587
Merge pull request #1602 from ghost1227/bugs/1587

Fix for issue #1587
Fix for issue #1585
Merge pull request #1603 from ghost1227/bugs/1585

Fix for issue #1585
Check for numeric keys when sanitizing settings to be compatible with extensions that have registered settings with numeric keys. Closes #1605
Merge pull request #1586 from polevaultweb/master

Adding filters to admin script loader
add fallback for cal_days_in_month. Closes #1601
stripslashes(). #1582
Simplify cart tax label display. #1604
remove  param in edd_get_cart_subtotal() and check if prices include tax before calculating taxes again. #1604