Check for DATAVERSE_SERVER

[andybega]

library(icews)
foo <- dataverse::get_file(2711073, dataset = get_doi()$historic)

doesn't work out of the box when the DATAVERSE_SERVER environment variable is not set. This is required by the dataverse R package and is mentioned in their readme, but that's not going to be apparent for anyone coming to icews.

• add Sys.setenv("DATAVERSE_SERVER" = "dataverse.harvard.edu") to README example
• add a function or something to check for the presence of this var when the package is loaded