Add Phonebook#dialpad_search

Given a phonebook, we can now search it for name/number pairs based on
input from a dialpad. We look for exact matches that starting a first or
last name.

This is perhaps inefficient, in that we could precompute the search
index and do a direct lookup, but this is certainly the quick and dirty
way to code this feature.
Specify Ruby 2.0.0 in Gemfile

Specifying a Ruby version is necessary for some PaaS deployment
strategies, and will help keep the application consistent across
environments.
Add Rackup file

Instructions on running the application. Used for easy deployments
(Heroku, Dokku, ...)