Bookmarks via Keypress

This PR adds the functionality to add ```<KEY>createbookmark</KEY>```
  or ```<KEY mod="ctrl">createepisodebookmark</KEY>``` to your keymaps.

Idea is from here: http://forum.xbmc.org/showthread.php?tid=83178

I also added a commit that adds 
```xml
<b>createbookmark</b>
<b mod="ctrl">createepisodebookmark</b>
```
to the default keyboard.xml

I know that ```<b>``` is already used for some PVR related stuff, so not sure if I should remove that commit again. But for now I left it in for easier testing.