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.

Omit unnecessary files in coveralls.
Nosetests will report any executed code for coverage.
Skips those files using the .coveragerc setting.

https://github.com/coagulant/coveralls-python#nosetests

Add pypy/lib_pypy to .coveragerc's omit section.
pypy/lib_pypy/ files were causing a lower coverage result
in coveralls.