Add event handling functionality

[fjarri]

Fixes #10

• support event declarations in the ABI (added a corresponding Event) class
• add eth_new_block_filter(), eth_new_pending_transaction_filter(), eth_new_filter(), eth_get_filter_changes() and the corresponding entities (BlockInfo, TxInfo, Topic, LogEntry)
• API for high-level event filtering

[codecov-commenter]

Codecov Report

Merging #32 (74b30d0) into master (0fce3ce) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #32    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            9         9            
  Lines          949      1261   +312     
  Branches       174       246    +72     
==========================================
+ Hits           949      1261   +312     

Impacted Files	Coverage Δ
pons/__init__.py	100.00% <100.00%> (ø)
pons/_abi_types.py	100.00% <100.00%> (ø)
pons/_client.py	100.00% <100.00%> (ø)
pons/_contract.py	100.00% <100.00%> (ø)
pons/_contract_abi.py	100.00% <100.00%> (ø)
pons/_entities.py	100.00% <100.00%> (ø)

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