Pagination / Pager code review and better customizing

Reviewed Pagination and Pager.

For Pagination:
* renamed variables to be conform with variables naming scheme (add '-default')
* add new variables for better customizing and states support (hover, active, disabled) 
* rewrite rules for better customizing and states support (hover, active, disabled) 
* use mixin ".pagination-size" also for base size
* removed z-index setting, because I did not see the need for

For Pager:
* removed variables `@pager-disabled-color`, because Pager use for all other color setting the variables of Pagination and the value of this variables was also the same that corresponding Pagination variables has
* add/rewrite rules for better customizing and states support (hover, active, disabled)