Stapes extended

Added new methods (based on [underscore.js](http://underscorejs.org) source, tests are included):

**pluck**
> extract a list of property values

**sortBy**
> returns a sorted copy of list, ranked in ascending order by the results of running each value through iteratee (function)

**groupBy**
> splits a collection into sets, grouped by the result of running each value through iteratee (function)

**countBy**
> sorts a list into groups and returns a count for the number of objects in each group