Add geometry update support

[luizguilhermefr]

Unfortunately, I still didn't find a way to support update on multiple models, as I cannot access getPostgisType() from a bulk Builder. However, I was able to add support to single model update.

Example:

$model = PostgisModel::find($id)
$model->point = new Point($x, $y);
$model->save();

Tests are passing, please let me know if something is missing.

Fixes #100

[njbarrett]

Thanks for your time on this, I believe it's an improvement even without the multiple model support currently.