Basic extractor classes and parse function + tests

Very basic implementation of parse API function which
iterates through all Extractor subclasses and extracts the
dates from the text and returns a list of these dates.

Has a christmas extractor, which extracts words like
christmas and christmas eve.

Includes test cases for all code. Test cases can be run
with the following command:
nosetests --cover-html --cover-html-dir=htmlcov --with-coverage

Add travis-ci.org and coveralls.io config for displaying tests
and coverage reports on the site.