(#117) Refactor EntrySpec

[llorllale]

(NEW) Xml interface
(NEW) Xml.asString()
(NEW) MockTimeTrackEntryType
(NEW) EntrySpec overloaded constructors
(NEW) EntrySpec.equals() and hashCode()
(NEW) TimeTrackEntrypType.equals() and hashCode()
(REF) IssueSpec: no longer holding nullable attributes inside
        an Optional
(REF) EntrySpec.asHttpEntity() renamed to EntrySpec.asXml()
(REF) XmlObject renamed to XmlOf (implements Xml)
(REF) StringAsDocument: now throwing UncheckedException instead
        of ParseException
(REF) XmlObjects renamed to XmlsOf (implements Collection<Xml>)
(FIX) Missing unit tests for EntrySpec
(DEL) ParseException: no longer used. It was being used in places
         where UncheckedException is appropriate.
(DEL) EntrySpec.with(Field, FieldValue): substituted by new
         constructor overloads

closes #117

[codecov]

Codecov Report

Merging #118 into master will increase coverage by 1.41%.
The diff coverage is 90.78%.

@@            Coverage Diff            @@
##           master    #118      +/-   ##
=========================================
+ Coverage   89.49%   90.9%   +1.41%     
=========================================
  Files          49      48       -1     
  Lines         676     704      +28     
  Branches       30      32       +2     
=========================================
+ Hits          605     640      +35     
+ Misses         63      56       -7     
  Partials        8       8

Impacted Files	Coverage Δ
...java/org/llorllale/youtrack/api/DefaultFields.java	100% <ø> (ø)	⬆️
...va/org/llorllale/youtrack/api/DefaultComments.java	100% <ø> (ø)	⬆️
...in/java/org/llorllale/youtrack/api/XmlComment.java	100% <ø> (ø)	⬆️
...in/java/org/llorllale/youtrack/api/XmlProject.java	84.21% <ø> (ø)	⬆️
...va/org/llorllale/youtrack/api/XmlUsersOfIssue.java	100% <ø> (ø)	⬆️
...main/java/org/llorllale/youtrack/api/XmlIssue.java	94.28% <100%> (+0.73%)	⬆️
.../llorllale/youtrack/api/XmlTimeTrackEntryType.java	100% <100%> (ø)	⬆️
...c/main/java/org/llorllale/youtrack/api/XmlsOf.java	100% <100%> (ø)
...va/org/llorllale/youtrack/api/DefaultProjects.java	100% <100%> (ø)	⬆️
...va/org/llorllale/youtrack/api/XmlProjectField.java	100% <100%> (ø)	⬆️
... and 12 more

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