lexRankr 0.5.0

• bug fix in sentence parsing for parsing exclamatory sentences
• converted idf calculation from idf(d, t) = log( n / df(d, t) ) to idf(d, t) = log( n / df(d, t) ) + 1 to avoid zeroing out common word tfidf values
• removed dplyr, tidyr, stringr, magrittr, & tm as dependencies
• created option to bypass assumption that each row/vector-element are different documents in lexRank and unnest_sentences
• various bug fixes in token & sentence parsing