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
thats the problem, it is saved to the database, as string with withspaces (!!!). :-(  I've added these constants to avoid typos and errors in our code. 

Next step could be the migration to a clean naming like "dynamic_price", "default_price", "category_price", "individual_price". 

_Note_ These hardcoded strings are also used somewhere in JS-Files..