* Created Column module and moved merge_duplicate_contacts into it.
  Takes all duplicated contacts of a certain type (email, phone) and
  merges together duplicate entries into a single entry.

* Wrote tests for Column.

* Moved a bunch of methods to Util module.
  - Created add_hash_value to Util for hashes where the values are
  arrays. When adding a new key-value pair to the hash, adds the
  value to the end of the values array if the key already exists.
  Otherwise creates a new array.
  -  Rewrote join_hash_values (also for hashes where values are
  arrays). If the hash value is unique, set the key to the hash value.
  Otherwise, join the hash values together.