Add model's withCount columns ot exempt attributes

[denitsa-md]

Pull request for #16 - exclude count columns from cloneable attributes

I wanted to ignore the count columns that also exist as explicitly set columns on the table by doing a diff (e.g. if a column randoms_count exists on the table and the column is also included in the $withCount = ['randoms'], it should still be included in the cloned attributes.)

But I'm not sure how to access the connection from inside the package.

$app['db'] inside Schema::connection() (facade) is NULL so I cannot call

Schema::connection($this->getConnection()->getName())->getColumnListing($this->getTable()) inside a function.

    private function getCountColumnsToExclude()
    {
        $count_columns = array_map(function($count_column) {
		    return $count_column . '_count';
        }, $this->withCount);

	return array_diff($count_columns, Schema::connection($this->getConnection()->getName())->getColumnListing($this->getTable()));
    }

[coveralls]

Coverage increased (+0.3%) to 91.753% when pulling 23611b6 on SubscriptionPeople:Add-withCount-model-columns-to-exempt-columns into 14a36bd on BKWLD:master.