Add the ability to use custom key and weight selection functions.
Incorporate the custom key and weight selection function arguments into the opts argument of Pickup's pick and Pickup's, CircleIterator's and MappedList's initialize methods.
Combine MappedList's uniq parameter with its opts hash while maintaining backwards compatibility (but with a deprecated warning).
Only show the deprecated warning if a boolean has been passed to MappedList's initialize method explicitly.
Merge pull request #1 from SanderKleykens/custom-key-weight-selector

Add the ability to use custom key and weight selection functions.