recreate and explore basic array methods
- re-implement: Push, Pop, Shift, and Unshift for Array.prototype, without using any library functions. Also add some Mocha tests to verify correctness.
- unique: implement a function called unique. It takes an array and returns a copy of the array with all the duplicates removed.
- frequency2: given an array of english words, figure out what the most common letter (anywhere in a word) is.