Added simple expression evaluation support as well as variable definition.

You can do this now:

<!-- set variable "hourly_rate" to dyn attribute 'hourly_rate' 
     or set it to the default "100.0" -->
<r:set var='hourly_rate'><r:show attr='[d_hourly_rate]'/> 100.0</r:set>
<r:data>
  <r:set var='hours' do='stat' find='sum'/>
  total price: <r:show var='hours'/> * <r:show var='hourly_rate' format='%.2f'/> = <b do='show' eval='hours * hourly_rate' format='%.2f $'/>
</r:data>

Have a look at set for details.