Refactor Task class and its dependencies to match current version

[michael-setia]

Fixes #213

Added the Task class that will represent the task in the TodoList.

1. New file added :

• Task class
• Date class
• Priority class (enum)
• Type class (will be replaced with tags in the future)
• TaskName class
• Status class (enum)
  ------ [updated 17/10/2020] -------

2. The following files have also been added (taken from future PR) since I just realized they are also the base classes for TodoList:

• DateComparator class
• TaskComparatorByDate class
• TaskComparatorByName class
• TaskComparatorByPriority class
• TaskNameComparator class
• Criterion enum

3. The following files are updated:

• TaskName class:

  • isSameTaskName() method. It now ignores the case for the string compared (uppercase or lowercase is the same)

• Priority class:

  • Added message constraints
  • Added isValid() method() [not used at the moment]

[codecov-io]

Codecov Report

Merging #226 into master will decrease coverage by 1.04%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #226      +/-   ##
============================================
- Coverage     21.50%   20.46%   -1.05%     
  Complexity      300      300              
============================================
  Files           178      190      +12     
  Lines          2734     2873     +139     
  Branches        227      242      +15     
============================================
  Hits            588      588              
- Misses         2105     2244     +139     
  Partials         41       41              

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/seedu/address/model/task/Criterion.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
src/main/java/seedu/address/model/task/Date.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...c/main/java/seedu/address/model/task/Priority.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
src/main/java/seedu/address/model/task/Status.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
src/main/java/seedu/address/model/task/Task.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...c/main/java/seedu/address/model/task/TaskName.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
src/main/java/seedu/address/model/task/Type.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../address/model/task/comparator/DateComparator.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ss/model/task/comparator/TaskComparatorByDate.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ss/model/task/comparator/TaskComparatorByName.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
... and 14 more

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[jonasngs]

The code is readable and understandable. Apart from a few missing javadocs, it looks good to merge.