Make aiohttp ClientSession optional

[bachya]

Describe what the PR does:

This PR makes the aiohttp ClientSession an optional parameter when creating an API object.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codeclimate]

Code Climate has analyzed commit a0779f2 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #28 into dev will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev       #28   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          142       148    +6     
=========================================
+ Hits           142       148    +6     

Impacted Files	Coverage Δ
aioflo/api.py	100.00% <100.00%> (ø)

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