Fixed header

Hi, these commits add classes to the three states(to eventually be used by css),  adds an overflow:auto to pinned content that is higher than the window, and support for a fixed header.

Last thing to be corrected, still, is to correct the following behavior: when a pinned content is in overflow, and the end of the container is reached, the pinned content should change state as soon as its normal height hits the bottom, not its current height(the overflow:auto one)