Fix issues reported by PHPStan

[jaylinski]

Fixed all PHPStan issues up to level 4 and added PHPStan to
CI scripts.

Added a new test-case to test a basic setup. This also tests the
removal of empty elements.

Fixes #27

[codecov]

Codecov Report

Merging #28 (9b4a729) into master (b90c0da) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #28   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       130       133    +3     
===========================================
  Files             33        33           
  Lines            378       376    -2     
===========================================
- Hits             378       376    -2     

Impacted Files	Coverage Δ	Complexity Δ
src/Tv.php	100.00% <ø> (ø)	6.00 <0.00> (ø)
src/Tv/Channel.php	100.00% <ø> (ø)	8.00 <0.00> (ø)
src/Tv/Elements/Audio.php	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
src/Tv/Elements/Subtitles.php	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
src/Tv/Elements/Video.php	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
src/Tv/Programme.php	100.00% <100.00%> (ø)	30.00 <0.00> (ø)
src/XmlElement.php	100.00% <100.00%> (ø)	18.00 <11.00> (+2.00)
src/XmlTv.php	100.00% <100.00%> (ø)	17.00 <7.00> (+1.00)

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