Adds prefix_slug method
Prefix taxonomy slug to minimize colissions
Uses taxonomy and CPT constant to minimize errors
prefix_slug can now prefix with plugin_name also
Return WP_Error if register_plugin fails
Modifies add_entry

If WP_Post is passed as $log parameter, then log already exists. If,
string is passed, create log post. Associate message as post meta with
the log.
Modifies method headers
Adds first version of WP_Logger_API library for plugins