little modif in gettemplate.

little modif in gettemplate.
auto-replaces theme and lang template variable.
This is useful if you have images in your template :
<img src="/<TMPL_VAR name="theme">/<TMPL_VAR name="lang">/images/picture.gif>
or
<img src="/<TMPL_VAR name="theme">/images/picture.gif>
in the case of a non-language dependant image.

Before this fix, gettemplate only replaced a variable called themelang, which was useful for language dependant images, but didn't work with theme-only images.
Note the previous behaviour is still activated (ie : themelang still works)