read_dfr_wordcounts rewrite

refactored into read_dfr and docs_frame so that it will be possible to
do some operations based on wordcounts before inflating the bags
However, stacking lots of dataframes, instead of just processing them
immediately, is terrible in R. Tried to do some optimizing.

also: removed some superfluous library(mallet) calls
read_metadata for multiple files

read_metadata now accepts a vector of filenames and rbinds and uniques
the result

read_citations is the former read_metadata, and it reads a single
citations.CSV (called repeatedly by read_metadata). read.citations is
deprecated.
comments in topics.R
Some documentation for each of the functions
script to source all R scripts here