Reset for slider and pagination

Pagination:
_reset: remove old pagination and generate new one from scratch.
In case of adding new items at the end of slides-container and removing
old ones from the beginning, pagination._addItem() from
superslides.update() is generating wrong numbers and pagination isn't
working correctly.

Core Prototype:
reset: The same as update(), but with pagination reset and position set
to first item.
Usefull when you want to have one slider with multiple galleries in it,
and you want to start from the beginning after switching slider
contents.