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
ah ok! Sorry my fault..i misunderstood you. 

Replaced the `self::` with `LaterPay_Helper_Pricing::' in https://github.com/laterpay/laterpay-wordpress-plugin/commit/60fadf4c1667b6103d9d411f99389482f992644b

Is this actually a Helper-Class and not a Model-Class?