Adding a CITATION file

Based on arguments in:
http://www.software.ac.uk/blog/2013-09-02-encouraging-citation-software-introducing-citation-files
Merge pull request #110 from ethanwhite/citation-file

Adding a CITATION file
Adding citation command. `retriever citation` gives the citation for the
retriever itself; retriever citation [dataset] gives the citation for a specific
dataset.
Look for citation data file in the correct place, not the working directory.
Merge pull request #112 from bendmorris/master

Add citation command to get citation info for the retriever itself or for individual datasets.