Created manager.py, a global manager class that takes lists of tweets or a JSON file and provides methods to create lists and dictionaries for
vectorization and other purposes.
Moved tweets_from_json and users_from_json from parsed.py to retrieval.py.
Modified parsed.ParsedTweet.mentions to combine results of ParsedTweet._mentions() and ParsedTweet._ats().
Added .split_hashes attribute and corresponding ._split_hashes method, which uses feature_extraction.freq_splitter.split_text to split
hashtags.