Implement the ability to append labels to the testcases and suites & some other changes.

* Implement the ability to append labels to the testcases and suites. So you can do the following: 
```ruby
describe MySpec, :feature => "Some feature", :severity => :normal do

  it "should be critical", :story => "First story", :severity => :critical do
    # ...
  end

end
```
* Validation. The resulting xml files are validated against the allure.xsd
* Tiny refactoring.