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
`$post_price['type']` will query a non-existent key here if `$post_price` was initialized in line 112.

Also, I don't get what this code is trying to do... why initialize `$post_price` in the first place if it's not used further. Did you mean to set `$post_price['type'] = self::TYPE_DEFAULT_PRICE` before?