Add new IExpressionPrinter interface, with CSharpExpressionPrinter implementation
Add Expression property in Field, allows a field to have a "value"
Pass in an CSharpExpressionPrinter instance to CSharpTextTemplate
Add support for fields with expressions (values) in CSharpTextTemplate
Merge pull request #168 from InitialForce/feature_ast_add_field_value

Add support for a expression type in the AST and make it possible to specify expression/value for a field.