Versatile Sentence Boundary Detection

[lecoqnicolas]

1. Created property sdb_package, with values either pointing to SDB subdirs (stanza/language-specific spacy) or None, in package
2. Created class StanzaSentencizer, rewrote SpacySentencizerSmall and base-class to be package-dependant and load from previously described property, or if None, from cache, in sbd
3. Fixed spacy automatic download: created function cache_spacy in networking
4. To avoid circular import, called the classes upon initializing the sentencizer in translate,
5. Fixed byte-fallback bug whilst kept active underscores rewritten as spaces (commented legacy code) in tokenizer
6. Commented all legacy code relative to "stanza_available" environment variable in settings, package and translate.
7. Swapped a few lines and edited spelling things here and there for consistency.

[lecoqnicolas]

Hello,
If you deem necessary to clean the legacy code or revert a feature before merging, please tell me next week, I'll do it.