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 personally think that `self` just makes refactoring a pain and adds no benefits unless you desperately want to make sure that you never call a overloaded method and I can't think of any valid use for that either. But if you ever move that code out of this class, it won't work anymore. I'd rather see us not use `self` anywhere. Any particular reason why we're using it here?