LUA HTTP Interface: Implementation of missing seeking funtionality when using status.xml from the lua http interface.

Seeking is now possible with relative values instead of just absolute ones.
Also the time format as mentioned in the readme file is now recognized:
[+ or -][<int><H or h>:][<int><M or m or '>:][<int><nothing or S or s or ">]
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>