changed file reader classes -- refactoring
minor change of FileReader class -- deleted unused import statements in the class
changed file reader classes -- changed type of field 'skipColumns': Set<Integer> to Set<String>
changed file reader classes -- added comments
changed file reader classes -- moved 'setSkipColumns' method to FileReader class
implemented JSONFileReader class
changed file reader classes -- changed parameters of 'initializeConvertedRow' method
minor change of JSONFileReader class -- deleted unused import statements from the class
changed JSONFileReader class -- fixed bug that it cannot parse JSON data
added integration test for JSONFileReader class
renew dataset for integration tests
minor change of JSONFileReader class -- added comment