Fixed automatic background updates not working for plugins that use this library.

Background (i.e. unattended) plugin updates require an additional $update->plugin field to be set to plugin file name relative to the /wp-content/plugins directory. Fixed by adding a $filename property to the PluginInfo and PluginUpdate classes and updating toWpFormat() to return it as "plugin". Apparently WordPress update API introduced the field a couple of minor versions back, but I didn't notice because it's used *only* by the background updater and ignored otherwise. 

I named the internal variable "filename" instead of "plugin" because "plugin" is very vague. It doesn't really tell you if the variable contains a plugin slug, plugin name, the full plugin filename, a partial filename, basename or something else. "filename" is a bit less ambiguous, though not perfect.

Bumped version to 1.6.