Model: Macros should be case insensitive

This patch fixes two problems with macro resolving:

* When asking for a custom variable it should be
  Case insensensitive (i.e. $_SERVICE_MACRO1 == service.__macro1)

* When calling get_all_macros() all macronames are returned in
  uppercase.
Model: Use Utils.is_macro() to test for macro
Fix undefined variable name
Merge pull request #177 from pynag/fix-customvars

Fix: Macros should be case insensitive