properties and macros in (typically 3rd party) include files

- enable the new behaviour by passing --inorder cmdline option
- to improve code readibility and reusability, introduced functions
  process_include(node), grab_macro(elt, macros), grab_property(elt, symbols)
  containing 1:1 corresponding handling from process_includes, grab_macros, and grab_properties
- added corresponding test case test_inorder_processing()

- replaced handle_expr with python-internal eval() call
- care has been taken to resolve variables recursively on demand (in Table.__getitem__)
- allows for evaluation of standard math functions
- other desired functions could be added in eval_self_contained
- Values in Table symbols are not stored as strings but as typed values.
  If text is required, a conversion with str() is performed.
  This is need to ensure proper evaluation of expressions. Otherwise 3*"1" would evaluate to "111".
- using __future__.division we can handle int division evaluating to float as before

- for equality expressions in <xacro:if>
- math function usage

- literal evaluation should only consider literals, but no expressions
  use ast.literal_eval()
- removed eval() from xacro:if evaluation
  back to string comparison to handle (lowercase) true and false