settings refactor

Hey Pippin,

I made some changes to the admin settings registration and display files.

Basically, I added a new action in display-settings.php to allow for addons to add their own tab in the settings page.

I then altered the register-settings.php to cater for adding custom tabs and also refactored a lot of redundant code out. There is now an overall filter on $edd_settings called 'edd_settings_all' which is what you would hook into to add custom settings, rather then the tab specific edd_settings_misc filter.

So instead of registering each tab of settings separately, I changed it to be in a loop which reduces about 80% of the code needed. I also then had to create a new filter called edd_settings_title out of necessity to cater for each setting section title and hooked into that with a new method called edd_settings_title_render

I then had to alter the edd_get_settings function to cater for any new sections by use of a new filter edd_settings_sections.