Improve how settings are handled

[zaus]

Split 'settings' and 'services' to separate option fields, but include "optimized" form/service reference for faster checking (rather than looping all entries):

array( {form_plugin_prefix+form_id} => array( {list of services} ) )

Then we don't have to load all services until they're supposed to be used. Importance of this would be determined by results of analytics tracking in Issue #19.

Considered splitting up services into separate options (as they would if we go with Issue #20), but not sure how performant it is to fill the options table.

Refactor get_settings, get_services, save_settings, save_services. Consider adding validation filters (i.e. the list of allowed indexes to save), at least for settings.