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.
Simplify colorizing text links.

+ Color unchanged here.
Merge pull request #37 from johnjohndoe/feature/check-for-missing-xml-attribute

Let parsing fail when XML is invalid.
Merge pull request #39 from johnjohndoe/feature/maintainable-date-range

Increase maintainability for date range assignment.
Merge pull request #42 from johnjohndoe/feature/extract-event-background-color

Use central definition for event detail background color.
Merge pull request #43 from johnjohndoe/feature/enable-colored-text-links

Simplify colorizing text links.