Feature:  Allow unlimited days

I added a test to ensure that the days filter was works as expected.  I also added the ability to set `$days = 0` to return all lines in the log.  This has the benefit of setting `$days = 0` in `LogReaderTest.php` so you don't have to change the dates in the log file.

Also, I added a `.travis.yml` config file in case you'd like to use travis-ci.org for running tests.  You can see my branch's tests are ![Build Status](https://api.travis-ci.org/mcrumm/monolog-parser.png?branch=bug/day-limit)!