Fixed attr_reader

It wasn't passing the value parameter to define_attribute_reader
Allow define_attribute_reader to accept a single-key hash

Now 'attr_reader a:42' can be used instead of 'attr_reader a, 42'
Added first and last accessors to Model:DSL

For accessing the first and last elements in the Model while in a DSL
block
Allow extrusion builder DSL blocks to access their lexical scopes
Moved group and extrude commands into the DSL module

Because it's where they belong. The two commands rely on the Builder to implement build_group and build_extrusion. This makes it easier for various Builder's to override the default behavior.
Hacked Model to accept Geometry::Transformation arguments

This is nasty, but I'm holding off doing it right until Geometry::Transformation uses named arguments. When that happens, Model can simply ask the ::new method which arguments it accepts. This should all happen around the geometry v7 timeframe.
Added xtranslate for ignoring translate blocks
Added named arguments to the DSL’s translate command
Small README tweak