Adds classes pin-absolute pin-fixed, and scrolling when the content is too long

Use case is the following: when the pinned content is too high for the window, add overflow:auto so that the user can scroll within that content as well.
Adds support for a fixed header.

Adds an optional fixedHeaderSelector to the option, allowing to trigger the pin as soon as that header is hit.
adds support for fixed header
fixes end trigger
fixes end trigger
Corrects README again

sorry, i was working directly via github.com, don't know how to cancel a commit here.. :/
Merge pull request #16 from walidvb/master

Fixed header