Add unique identifier to Person class

[hhdqirui]

Add unique identifier to Person class. The field id is included to be used for identifying a unique person.

[hopinxian]

A few comments are made. After finishing them, this can be approved.

[codecov-io]

Codecov Report

Merging #127 into master will increase coverage by 0.48%.
The diff coverage is 87.50%.

@@             Coverage Diff              @@
##             master     #127      +/-   ##
============================================
+ Coverage     72.95%   73.43%   +0.48%     
- Complexity      690      716      +26     
============================================
  Files           106      107       +1     
  Lines          2074     2142      +68     
  Branches        233      247      +14     
============================================
+ Hits           1513     1573      +60     
- Misses          503      508       +5     
- Partials         58       61       +3     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/AddressBook.java	85.71% <50.00%> (-5.96%)	13.00 <1.00> (+1.00)	⬇️
...rc/main/java/seedu/address/model/ModelManager.java	88.88% <50.00%> (-1.64%)	41.00 <1.00> (+1.00)	⬇️
...java/seedu/address/logic/commands/EditCommand.java	95.34% <83.33%> (-0.95%)	12.00 <0.00> (ø)
...c/main/java/seedu/address/model/person/Person.java	95.00% <87.50%> (-3.04%)	37.00 <10.00> (+12.00)	⬇️
...a/seedu/address/model/person/UniquePersonList.java	94.64% <100.00%> (+1.95%)	30.00 <6.00> (+10.00)
...son/exceptions/PersonNotIdentifiableException.java	100.00% <100.00%> (ø)	1.00 <1.00> (?)
.../java/seedu/address/storage/JsonAdaptedPerson.java	100.00% <100.00%> (ø)	16.00 <0.00> (+1.00)

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[KohHanMing]

Looks good!

[hopinxian]

LGTM