Tests for parsers are added

Tests for full parser and simplified reference parser
are added. The new tests simultaneously act as documentation
for parsers. Also some mostly cosmetic changes are applied
to parser building functions and Parser() class.
diff --git a/requirements.txt b/requirements.txt
index 5b3aadd..5f6aed1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@
 pyyaml
 six
 enum34
+ddt