Symspellpy

- Updated `frequency_dictionary_en_82_765.txt` dictionary with common contractions
- Added `_below_threshold_words`, `_bigrams`, `_count_threshold`, `_max_dictionary_edit_distance`, and `_prefix_length` when saving to pickle. (closes [93](https://github.com/mammothb/symspellpy/issues/93))
- Implemented `to_bytes` and `from_bytes` options to save and load pickle with bytes string
- Updated data_version to 3
- Removed Python 3.4 and Python 3.5 support

- Removed numpy dependency
- `word_segmentation` now retains/preserves case.
- `word_segmentation` now keeps punctuation or apostrophe adjacent to previous
word.
- `word_segmentation` now normalizes ligatures: "scientific" -> "scientific".
- `word_segmentation` now removes hyphens prior to word segmentation
(untested).
- American English word forms added to dictionary in addition to British
English e.g. favourable & favorable.

- Modified `load_bigram_dictionary` to allow dictionary entries to be split
into only 2 parts when using a custom separator
- Added dictionary files to wheels so `pkg_resources` could be used to access
them

- Added `separator` argument to allow user to choose custom separator for `load_dictionary`

- Added `load_bigram_dictionary` and bigram dictionary `frequency_bigramdictionary_en_243_342.txt`
- Updated `lookup_compound` algorithm
- Added `Levenshtein` to compute edit distance
- Added `save_pickle_stream` and `load_pickle_stream` to save/load SymSpell data alongside other structure (contribution by [marcoffee](https://github.com/marcoffee))

- Added `transfer_casing` to `lookup` and `lookup_compound`
- Fixed prefix length check in `_edits_prefix`