refactor Gender.value from string to enum

[nicktohzyu]

couldn't get enum.toString to work, so for jsonification I changed Gender.value to Gender.toString (wanted Gender.value.toString)

[nicktohzyu]

closes #128

[codecov-io]

Codecov Report

Merging #159 into master will increase coverage by 0.10%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #159      +/-   ##
============================================
+ Coverage     26.47%   26.57%   +0.10%     
- Complexity      290      291       +1     
============================================
  Files           125      125              
  Lines          2312     2322      +10     
  Branches        276      277       +1     
============================================
+ Hits            612      617       +5     
- Misses         1661     1665       +4     
- Partials         39       40       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/storage/JsonAdaptedClient.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../seedu/address/storage/JsonAdaptedHairdresser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../main/java/seedu/address/ui/client/ClientCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../seedu/address/ui/hairdresser/HairdresserCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...c/main/java/seedu/address/model/person/Gender.java	63.63% <62.50%> (-11.37%)	7.00 <2.00> (+1.00)	⬇️

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[zyifanz]

LGTM!