add DOI link to citation styles (export layouts)

[Robert Jäschke]

see summary ;-)

a proper DOI link should look like this:

<a href="http://dx.doi.org/10.1016%2Fj.tics.2007.06.003">DOI: 10.1016/j.tics.2007.06.003</a>

or

<a href="http://dx.doi.org/10.1016%2Fj.tics.2007.06.003">DOI</a>