[pagination feature] add input box and allow dynamic resizing

Presently, the only way to customize the size of a page is by setting `stItemsByPage` on the `stPagination` directive; this attribute is read once during `postLink`, after which the value of `stItemsByPage` is ignored.

I would like `stPagination` to expose a way for users to dynamically change the number of rows displayed. This will probably involve one or more of

* binding an isolate scope property to an attribute and watching it
* allowing transclusions where a method to set the number of items is exposed to transcluded templates
* exposing a form input where a user can input the number of items per page

This is a quick (but ugly) implementation of the third one where a user can type in the number of items per page, but I think it needs refining before being included.