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
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
Fix pagination issue in history
Simplify cart tax label display. #1604
Added filetype check for settings import
remove  param in edd_get_cart_subtotal() and check if prices include tax before calculating taxes again. #1604
Switch to edd_get_file_extension()
Merge pull request #1608 from ghost1227/bugs/1526

Added filetype check for settings import
Hide Download Limit field from shop vendors
use edd_get_payment_use_id(). #1599
Fix syntax error
esc_url() and simplification of add/remove_query_arg()
Add check for apache on htaccess notification
Merge pull request #1611 from ghost1227/bugs/1593

Add check for apache on htaccess notification
Fix saving bug with vendor/download limit
Cleanup redundant remove_query_args()