More unit tests on newest methods
Refactor BaseCollector._expand_wildcards method

 - Iterate over a set created from the list passed as an argument.
 - Raise an exception if the item is negated and not negated.
Refactor BaseCollector._format_wildcard method

Do not use lambdas, but use normal functions so as to be able to
manipulate the strings that we are returning. Otherwise the tuples were
returned as ("foo",) that causes mysql to fail.
Refactor Report._report_from_yaml and add unit tests

Make the YAML checks on Report._report_from_yaml instead of doing them
in the collect method. Add unit tests for proper parsing.
Move BaseCollector._to_hours method to utils
Use fixtures to store valid filters for collectors
Merge pull request #20 from alvarolopez/unit_tests

Refactor some code and add more unit tests