결제사전검증(prepare) api 추가

[bskim45]

payments확장기능으로 결제 사전검증(prepare) 관련 api를 추가하였습니다.

• api 추가
• test 추가
• readme 반영

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.81%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #17      +/-   ##
=========================================
+ Coverage   90.09%   90.9%   +0.81%     
=========================================
  Files           2       2              
  Lines         101     110       +9     
  Branches       14      14              
=========================================
+ Hits           91     100       +9     
  Misses          8       8              
  Partials        2       2

Impacted Files	Coverage Δ
iamport/client.py	90.74% <100%> (+0.84%)	✅

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[coveralls]

Coverage increased (+0.6%) to 92.727% when pulling 5dc0ba2 on bskim45:master into 6e5eb76 on iamport:master.

[coveralls]

Coverage increased (+0.6%) to 92.727% when pulling 5dc0ba2 on bskim45:master into 6e5eb76 on iamport:master.

[coveralls]

Coverage increased (+0.6%) to 92.727% when pulling 5dc0ba2 on bskim45:master into 6e5eb76 on iamport:master.

[coveralls]

Coverage increased (+0.6%) to 92.727% when pulling 5dc0ba2 on bskim45:master into 6e5eb76 on iamport:master.

[perhapsspy]

기여 감사합니다.