catmaid R package v0.8

This is a major version update because of the new support (and recommendation)
to use environment variables for login (.Rprofile is still supported).

• support fetching login informaion from environment variables (#80)
• catmaid_get_compact_skeleton now returns list of vectors (#79)
  (used to be a list of lists, but this was less convenient and not helpful)
• fix query_by_neuron_or_annotation adding empty annotations attribute