Updates to make package compatible with Laravel 5.1

[anaxamaxan]

Main changes in this pull request:

1. Update DbExportHandlerServiceProvider::boot() and ::register() methods to use the L5 config publishes() style. This allows the config to get published via php artisan vendor:publish.
2. Remove the $this->package('nwidart/db-exporter'); call from DbMigrationsServiceProvider::boot() method. It's enough to have it publishes() call in the export handler, so leave it empty.
3. Register \Str alias in DbExportHandlerServiceProvider::loadAlias() because some users won't have \Str alias registered in their config/app.php, and you're already going the extra mile, so might as well do it for \Str too.
4. Replace config strings using L4 style, e.g. Config::get('db-exporter::export_path.seeds') --> config('db-exporter.export_path.seeds')
5. Replace Config::get() with config() just to be more consistent with Laravel docs.

[anaxamaxan]

oops, there's an incorrect config path... I'll resubmit this PR