Amend deadline to use DateTime util class; Create test cases for Deliverables

[gabztcr]

-Now to get string of Deadline, we use getDeadline().toString() and not getDeadline().value anymore. Fixes #86
-A truckload of test cases, for now. #62

[codecov-io]

Codecov Report

Merging #112 into master will increase coverage by 6.75%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master     #112      +/-   ##
============================================
+ Coverage     47.39%   54.14%   +6.75%     
- Complexity      606      679      +73     
============================================
  Files           153      154       +1     
  Lines          2817     2809       -8     
  Branches        324      322       -2     
============================================
+ Hits           1335     1521     +186     
+ Misses         1380     1169     -211     
- Partials        102      119      +17     

Impacted Files	Coverage Δ	Complexity Δ
...address/logic/commands/deliverable/AddCommand.java	100.00% <ø> (+100.00%)	8.00 <0.00> (+8.00)
...ress/logic/commands/deliverable/DeleteCommand.java	100.00% <ø> (+100.00%)	7.00 <0.00> (+7.00)
.../address/logic/commands/meeting/DeleteCommand.java	100.00% <ø> (ø)	7.00 <0.00> (ø)
...du/address/logic/commands/meeting/EditCommand.java	82.27% <ø> (ø)	12.00 <0.00> (ø)
...ess/logic/parser/deliverable/AddCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...edu/address/model/deliverable/DeliverableBook.java	87.50% <ø> (+87.50%)	11.00 <0.00> (+11.00)
...java/seedu/address/model/meeting/meeting/From.java	60.00% <ø> (ø)	4.00 <0.00> (ø)
...ss/storage/deliverable/JsonAdaptedDeliverable.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...rc/main/java/seedu/address/ui/DeliverableCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...java/seedu/address/ui/DeliverableDetailsPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 17 more

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[shadowezz]

LGTM, just some minor comments.