Update Encoder specification; Bump version to 0.4.1

[PetrochukM]

Updates:

• enforce_reversible specification has changed from a function to a property.
• spacy.batch_encode now consistently returns length along with the encoded object.
• More code reuse through object orientated programming.
• Loosened up the specification for Encoder to return objects rather than Tensors.

[codecov-io]

Codecov Report

Merging #69 into master will decrease coverage by 0.37%.
The diff coverage is 82.66%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   94.33%   93.95%   -0.38%     
==========================================
  Files          57       57              
  Lines        1518     1523       +5     
==========================================
- Hits         1432     1431       -1     
- Misses         86       92       +6

Impacted Files	Coverage Δ
torchnlp/encoders/text/whitespace_encoder.py	100% <ø> (ø)	⬆️
torchnlp/encoders/label_encoder.py	94.11% <100%> (+1.52%)	⬆️
torchnlp/encoders/text/text_encoder.py	100% <100%> (ø)	⬆️
torchnlp/__init__.py	100% <100%> (ø)	⬆️
torchnlp/encoders/text/subword_encoder.py	97.43% <100%> (+0.21%)	⬆️
torchnlp/encoders/text/static_tokenizer_encoder.py	97.87% <100%> (+0.31%)	⬆️
torchnlp/encoders/text/treebank_encoder.py	70.58% <50%> (-8.36%)	⬇️
torchnlp/encoders/text/moses_encoder.py	66.66% <60%> (-8.34%)	⬇️
torchnlp/encoders/text/character_encoder.py	83.33% <66.66%> (-6.67%)	⬇️
torchnlp/encoders/text/delimiter_encoder.py	85.71% <66.66%> (-5.96%)	⬇️
... and 2 more

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