Implement duplicate contact handling

[brianchoon]

Summary

Implement duplicate contact handling based on the following criteria:

Two contacts are considered duplicates if any one of the following criteria holds true

• They have the exact same phone number
• They have the exact same email

Having the same name alone is not enough to be classified as a duplicate.

Issues Fixed

Fixes #58

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
...c/main/java/seedu/address/model/person/Person.java	100.00% <100.00%> (ø)	22.00 <0.00> (+1.00)