Merge pull request #2 from rinatkhaziev/master

Updated readme and config-sample.php with explanation of validation_cb and new MultipleCheckboxesOption
Use strict null check for non-existant options when building output

Was creating a bug for Boolean options with a 0 default because the `! get_option( id )` would always return true. Both entries in the boolean dropdown would get the "selected" attribute.
Fix a warning produced when second parameter for in_array check was actually a string instead of array. This check is needed to determine if a checkbox in a set of values for multiple checkboxes is selected but only one or no values were set, resulting options_checked variable being a string instead of an array. Cast to array before doing the check