Rules for the Exsurgent Virus added

The rules for Exsurgents are now available in the devel-branch. They will be in the 0.6 version of the pdf, but for now, you can take a look at the file in the repository and comment on what you think about the rules. Adding the rules for the Exsurgent Virus was one of the 4 milestones for the next release, which means that there are only 3 to go until we get to version 0.6.