Adds setting ETHEREUM_LOGS_FILTER_GETLOGS.

[xorader]

This option enables using web3.eth.getLogs(...) function instead
web3.eth.filter(...).get_all_entries().

[codecov-commenter]

Codecov Report

Merging #17 (5cca910) into master (46cc4cc) will decrease coverage by 0.40%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   81.51%   81.11%   -0.41%     
==========================================
  Files          13       13              
  Lines         357      360       +3     
  Branches       53       54       +1     
==========================================
+ Hits          291      292       +1     
- Misses         50       52       +2     
  Partials       16       16              

Impacted Files	Coverage Δ
django_ethereum_events/event_listener.py	76.08% <20.00%> (-1.45%)	⬇️

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