Support for past data in epg timeline view

As @da-anda suggested this adds support for past data in timeline view. 
It's a work in progress, because not all backends (e.g. tvheadend) supports past epg data and i want to discuss how to handle it.

Currently i use a linger timer for epg data of 24 hours, so that you could go 24 hours back in time.

This also adds a new entry 'Go to now' into the context menu.

![screenshot001](https://f.cloud.github.com/assets/1878991/1599681/26662590-535d-11e3-939d-c2aa8d2aa8a0.png)