Improve support for software citations

[hubgit]

e.g. for <element-citation publication-type="software">, when <source> and <uri> are present, without <article-title> or <comment>:

<element-citation publication-type="software">
  <person-group person-group-type="author">
    <name><surname>Pinheiro</surname><given-names>J</given-names></name>
    <name><surname>Bates</surname><given-names>D</given-names></name>
    <name><surname>DebRoy</surname><given-names>S</given-names></name>
    <name><surname>Sarkar</surname><given-names>D</given-names></name>
    <collab>R Core Team</collab>
  </person-group>
  <source>nlme: linear and nonlinear mixed effects models</source>
  <edition designator="3.1-121">R package version 3.1-121</edition>
  <uri>http://cran.r-project.org/package=nlme</uri>
  <year>2015</year>
</element-citation>

[hubgit]

JATS 1.1 provides new version and data-title elements:

<element-citation publication-type="software">
  <person-group person-group-type="author">
    <name><surname>Pinheiro</surname><given-names>J</given-names></name>
    <name><surname>Bates</surname><given-names>D</given-names></name>
    <name><surname>DebRoy</surname><given-names>S</given-names></name>
    <name><surname>Sarkar</surname><given-names>D</given-names></name>
    <collab>R Core Team</collab>
  </person-group>
  <data-title>nlme: linear and nonlinear mixed effects models</data-title>
  <version>3.1-121</edition>
  <uri>http://cran.r-project.org/package=nlme</uri>
  <year>2015</year><!-- optional -->
  <comment>(R package)</comment>
</element-citation>