Added a fill function and extended the _.partial function

These four commits cover two extensions to underscore:
1. Addition of a new _.fill function: It's purpose is to fill a given sparse array with elements from another array. An optional options hash allows one to specify whether or not all filler items should be consumed and which elements are to be considered vacuous.
2. Extension of _.partial using the previous _.fill function: It is now possible to specify which arguments of the passed function should be partially filled in; we no longer need to start with the first argument anymore.

Please have a look at the tests for further clarification.