set the category-price to posts with default-price

* set category-price to post with default-price when first time a price is assigned to a category
* moved pricing-types to constants in LaterPay_Helper_Pricing
* replaced own implementation of `get_category_by_name`  with WordPress API [`get_term_by()`](http://codex.wordpress.org/Function_Reference/get_term_by)
* fixed table-name in CategoryPrice-Model 

__References to Issue:__ #231
I know what `self::` is used for, I'm just saying that we shouldn't use it. If you refactor and move this code to another file/class it'll fail unless the constants are defined in the exact same class (not even a baseclass, because that's how `self` operates). We should use full class references like `LaterPay_Pricing_Helper`. I can't think of a single instance where you would actually really want `self`'s side effects (namely sidestepping the class' "vtable")