Date is now saved as a String. No longer as a Date object.
newChallenges that are added to the text file are now deleted as soon as
they are unused (still collected by Garbage Collector, of course).
UUID added. Changed signature of Challenge constructor.
Modify how the challenge is created with the constructor.
challenge.java fixed. good versions of constructors
Changed file writing operation to be clearer and hopefully less buggy.
File writing changed in CreateAChallengeActivity.java

Date stored in Challenge.java as a string