Add walk module with versions of map, pluck, etc. for trees.
Add an entry point in package.json

This adds a file, `index.js`, which loads all of the mixins and exports
the modified underscore library. After setting the `main` property in
package.json to this index.js file, underscore with the contrib modules
mixed in can be included a project by doing:

`var _ = require('underscore-contrib');`
Rename bottomup => postorder and topdown => preorder, and eliminate aliases.
_.dictionary
Juxt now supports functions w/ >1 argument
Add test cases for mixed trees.
Merge pull request #27 from obmarg/juxt-arguments

Juxt now supports functions w/ >1 argument
Revise error message for things that aren't trees.
Merge pull request #25 from brianloveswords/node-module

Add an entry point in package.json
Author info
Merge pull request #26 from raganwald/master

Proposed: _.dictionary is the 'flip' of _.accessor
Merge pull request #24 from pdubroy/master

Add versions of the collection functions for trees and lists of trees.
Made _.walk available as a function in case others would like to use it as a building block.