Make more robust the period parsing. Add unit tests

Created by: epiclabsDASH

Partial refactor of getRegularPeriods method of DashManifestModel. Added a new check to cover situations in which multi period streams, for static assets, are not defining start and duration attributes of any of their periods. This avoids dash.js to throw exceptions, and logs a warning message to notify the user about the issue.

Note: Having this situation implies a wrong mpd and that, even with this fix, there will be timing inconsistencies after the first period. However, IMHO, I prefer dash.js to play something whenever is possible. Once we add the flag to dash.js to allow/disallow playback of streams/assets out of DASH IOP guidelines, I will revisit this.

Added unit tests to cover this and regular situations (multiple combinations of period with combinations of start and duration attributes).