Add simple Phonebook data structure

We are going to be trying to search through a list of contacts (which I
am calling a Phonebook for ease of discussion). We are going to store
these in some form; for now, the simplest possible answer is a hash
mapping Names to Numbers.

For the time being, we are going to consider phone numbers as strings;
this way we can accept arbitrary formatting (until we know better what
expectations Twilio has).
Teach Phonebook to translate words into dialcode

We want to be able to search from input given by a dialpad. This means
that we will want to index contacts by their dialpad equivalent ("Noah"
-> "6624") so that we can easily search our list.

To make this easy, here is a utility function: Phonebook.phoneword,
which maps a string to its digit equivalent.

Tests are implemented with RSpec (http://rspec.info/).