Remove unnecessary and incorrectly-used `$wpdb->prepare()` that triggers an error currently. Since no untrusted variables are added to the query, there's nothing to prepare.

See http://wordpress.org/support/topic/prepare-warning-in-35.
Remove another unnecessary use of `$wpdb->prepare()` for the same reasons as in 8369dc6.
Implement singleton pattern in place of reliance on a global variable.
Make class variables private and provide access via a magic getter.
Correct phpdoc implementation
If post counts are desired, ensure large numbers are probably localized for display.
Coding standards cleanup
Update readme to reflect changes in v0.3.
Note GitHub URL in plugin description.