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
Are we modifying a string here and then casting it to a float? Shouldn't this use `floatval`? It also needs error handling I think, seeing as it blindly casts a value from `$_POST` to `float`?