Add import:node task

Imports data for a node given as an environmental variable. For example:

  rake import:node NODE=industry_burner_coal technical_lifetime=14.0

Intended for use with node analysis documents to easily update
attributes from Excel VBA.
import:node task loads Atlas via Bundler

This is better than assuming that ETSource and Atlas both coexist in the
same parent directory.
Merge pull request #886 from quintel/import_node_analysis

Created import:node task