Rework the settings

Also add error handling for file operations and fix oddity when "resetting" the
view's color scheme to its previous value.
Fix errors about color schemes not being found

Because Sublime Text (2) calls `on_activated` when a plugin was loaded for ALL
views, including widgets and some invisible views or something there were way
more calls then necessary. Widgets were detected by the 'is_widget' setting but
the invisible "void views" could not and thus the dimmed scheme ended up being
assigned to them. However, when deactivating the plugin it would not iterate
over these invisible views resulting in errors when deactivating (or when
restarting and thus removing the scheme file in the process).

See: http://www.sublimetext.com/forum/viewtopic.php?f=6&t=11716&p=48146#p48146
Update the settings before calling the callback

This resulted in an infinite loop because the on_change event was fired when
the color scheme (or the theme) changes and since we didn't update the internal
settings it would still think that settings have been changed and thus do it
all over again.
Rename all the `grey`s to `gray` internally (including the setting!)
Update README and CHANGELOG