Refactor logic components

[bryanyee33]

Change AddressBookParser.java::parseCommand() to static command.
Remove CommandResult.java in favour of String results, to make it less error prone and more readable.

Updates #54

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 73.75%. Comparing base (3b04b21) to head (97b9b3c).

❗ Current head 97b9b3c differs from pull request most recent head 4d0d4af. Consider uploading reports for the commit 4d0d4af to get more accurate results

Files	Patch %	Lines
src/main/java/seedu/address/ui/MainWindow.java	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #60      +/-   ##
============================================
- Coverage     74.30%   73.75%   -0.55%     
+ Complexity      405      392      -13     
============================================
  Files            70       69       -1     
  Lines          1292     1269      -23     
  Branches        107      105       -2     
============================================
- Hits            960      936      -24     
- Misses          302      303       +1     
  Partials         30       30              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[aureliony]

LGTM

[yisiox]

LGTM