ST3 settings fixes

This addresses #467 (and possibly other issues) by adding user preferences to the settings reload. User preferences are now used if they exist, otherwise it falls back to the existing `SublimeLinter.sublime-settings` file.

It also fixes a hang that I assume was caused by the myriad clear_on_change / add_on_change calls when saving settings. Only one global listener is required on the global settings objects.