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
the `self::` is used for a constant. 
I've moved all hardcoded strings to these constants ( see here: https://github.com/laterpay/laterpay-wordpress-plugin/pull/269/files#r16758049 ). You can call them outside of the class with: `LaterPay_Pricing_Helper::TYPE_CATEGORY_PRICE`.