Added a dictionary facility to dfm()

See also testDictionary.R in the tests folder
updated dfm() with a hierarchical dictionary method

Thanks Kohei!!
with documentation updated for dfm() and functions
Fixed regular expression method in dfm()

Now it takes “wildcards” and converts them into regular expressions.
undid?? a change in subset.corpus.Rd?

-\method{subset}{corpus}(corpus, subset = NULL, select = NULL)
Merge branch 'master' into dev