Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
Update README.rst
Updated with reccomendation to use @allure.smth decorator instead of @pytest.allure.smth. Also typo with unmatched single quote was fixed.
Updated to be able to run test that have only "story" specified without "feature".
Merge branch 'upstream/master' into feature/features_and_stories

Conflicts:
	README.rst

Also code was refactored.
Fixed pep8.
Updated README.rst with example of multiple stories and features.
Reverted fix for stop_case() method also test was reverted.
Updated arguments for feature() and story() decorators.
Updated docstring for labels_of() method.
Tests for labels were updated.
Fixed pep8 for test_traces.py file.
Updated constants to handle different labels not only 'feature' and 'story'.
Updated pytest_plugins.py with features_label_type, stories_label_type and label mark.
Updated utils.py with get_marker_that_starts_with method to provide all labels through labels_of method and added LabelsList class with rules for list comparison.
Updated test_labels.py to use hamcrest matchers and new test for labels was added.
Fix for trace message was moved back as been removed by mistake.
Merge branch 'upstream/master' into feature/features_and_stories
Fixed import in test_xml.py
Fixed pep8 errors.
Updated with additional tests for labels.
Merge pull request #25 from F1ashhimself/feature/features_and_stories

Added support for Features and Stories.