Removed dependence to igraph0

* Plots are not supported in version 0.8.6 since igraph0 has been
  archived. We intend to resolve the issue in a future version.
  Install version 0.8.5 and igraph0 manually from the archive if
  plots are required. See section 6.3 in 'R Installation and
  Administration' on how to install packages from source.
Added .Rbuildignore

* Added .Rbuildignore with README.md. The readme file is only supposed
  to show on github