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.