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've fixed this here: https://github.com/laterpay/laterpay-wordpress-plugin/commit/e3611bd521d87ce8dd08d3b960d45b7d8ba91f5b . Thanks!

On this Method, we load all posts which are having an post price and looping through them. If the post price-type is the "global default price", than we change the price to the "category default price" with the `apply_category_default_price_to_post`-Method.

Maybe we should also move the "which post price-type has post_id {x}" to a own Method.