tests: adds new method for testing run_shell_command with xmllint
Utils: fix run_shell_command to support passing multiple args.
Elsevier: fixes wrong type of argument passed to the function
tests: improve test for run_shell_command for xmllint

* Changes parameters passed to xmllint,

* Adds unpacked Elsevier DTD to tests/data and sends the path to
  xmllint as argument.
Merge pull request #98 from Dziolas/xmllint_test

tests: adds new method for testing run_shell_command with xmllint