Attribute and method for specifying the behavior of repeated uses of single value options

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When an option for single value options (non-collection) is passed to the 
parser multiple times, the last value is kept.

Add an attribute and method for autowired and stand alone options which lets 
the user of the parser specify a behavior for repeated use.

  * RepeatBehavior.KeepFirst
  * RepeatBehavior.KeepLast (default)
  * RepeatBehavior.Error

When using "KeepFirst" with stand alone options, the handler should only be 
called for the first use.

Original issue reported on code.google.com by michael.praschl on 21 Mar 2013 at 7:25