Use pprint to pretty print variables (ie. dicts and lists)

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Data model improvements

* Propagate scope to nested Dict and Expression instances
* Generalize preparation and validation of Variable values
* Allow namespace to contain Expression instances
* Support immediate eval Expressions in assignment targets in parser

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Merge pull request #6 from esben/master

Improvements to data model