Update pkgDNLs_worldmapcolor

Updating pkgDNLs_worldmapcolor function with ggplot2::aes_string (better
for R CMD CHECK).
Also, the 'data' directory of the package is not on the CRAN (but is on
the github version of the package). pkgDNLs_worldmapcolor(...) needs the
'WorldBordersData.RData' file in order to work.
Merge pull request #15 from borishejblum/master

Updating pkgDNLs_worldmapcolor