Allow on_partition_assigned and on_partition_revoked to be async functions

[ask]

This is a backward compatible change, since it makes async optional.

We require these to be async functions since we need to call async code from within
the on_partitions_assigned and on_partitions_revoked handlers.

Note: Fixes #159

[codecov]

Codecov Report

Merging #190 into master will decrease coverage by 0.1%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   97.22%   97.12%   -0.11%     
==========================================
  Files          11       11              
  Lines        1872     1876       +4     
==========================================
+ Hits         1820     1822       +2     
- Misses         52       54       +2

Impacted Files	Coverage Δ
aiokafka/group_coordinator.py	93.84% <75%> (-0.36%)	⬇️

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[tvoinarovskyi]

Cool, looks good!