Let parsing fail when XML is invalid.

+ Check for missing 'end' attribute in 'day' element.
+ Avoid running DateFieldValidation when parsing already failed since
  an exception thrown by this class causes an application crash.
  Instead show a meaningful Toast message.
Increase maintainability for date range assignment.
Use central definition for event detail background color.

+ Allows easier customization.
Merge branch 'feature/maintainable-date-range' into datenspuren-2014
Merge branch 'feature/check-for-missing-xml-attribute' into datenspuren-2014
Merge branch 'feature/extract-event-background-color' into datenspuren-2014
Configure application for Datenspuren 2014.
Disable certificate check for Datenspuren 2014.
Turn background into brown for Datenspuren 2014.
Configure link for Google PlayStore for Datenspuren 2014.
Update link to repository for Datenspuren 2014.
Customize about dialog image for Datenspuren 2014.
Customize launcher icon for Datenspuren 2014.
Customize action bar icon for Datenspuren 2014.
Turn ActionBar into dark brown for Datenspuren 2014.
Customize launcher icon for Datenspuren 2014.
Customize spinner for Datenspuren 2014.

+ The spinner is used for day selection.
Colorize text links for Datenspuren 2014.
Customize conference URL for Datenspuren 2014.
Remove copyright notice for 30C3 artwork for Datenspuren 2014.