\Log::_init() - Extended determinig name and location of logfile by new ...

...config "log_filename"

log_filename = 'fuelphp.log' - for loging is used only this one file in log_path dir.
log_filename = null - use original way

I made this extension, because I want use linux logrotation

Signed-off-by: Kristián Feldsam <feldsam@gmail.com>