Update add entry behaviour

[epicfailname]

Resolves #42
Resolves #43
Resolves part of #19

Summary of changes

• Rename of Comment to Description
• Make Title and Description not compulsory fields when adding an Entry
  • Default field is the empty string
  • Empty string is to be seen the value reserved solely for developers
  • Users are disallowed from directly input the empty string in add or edit to an entry
  • This means that the field classes (e.g. Link, Description) have two separate validity regex checks for its String value
    • User input (parser)
    • Contruction
• Update behavior of Entry's isSameEntry() function
  • This function is mainly used to prevent duplicate entries in entry book
  • Previously used Title and (Comment or Link) for comparison
  • Change to now only use Link for comparison
• Update Link user-input and construction validation regex to be more permissive
  • For file paths, double forward slashes // to be optional
• Renames of more variables and function names to align with EntryBook context (not complete)

[coveralls]

Pull Request Test Coverage Report for Build 222

• 158 of 164 (96.34%) changed or added relevant lines in 27 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.4%) to 92.997%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/model/entry/Description.java	10	11	90.91%
src/main/java/seedu/address/model/entry/Entry.java	8	9	88.89%
src/main/java/seedu/address/storage/JsonAdaptedEntry.java	9	10	90.0%
src/main/java/seedu/address/logic/parser/ParserUtil.java	21	24	87.5%

Files with Coverage Reduction	New Missed Lines	%
src/main/java/seedu/address/logic/parser/ParserUtil.java	1	88.64%
src/main/java/seedu/address/storage/JsonAdaptedEntry.java	2	93.33%

Totals
Change from base Build 221:	-0.4%
Covered Lines:	1660
Relevant Lines:	1785

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💛 - Coveralls