Pay per request billing mode

[borgstrom]

This PR continues on #72 to add PAY_PER_REQUEST billing mode support to our table objects.

FYI @mgwilliams

Checklist

• Tests have been written to cover any new or updated functionality
• All tests pass when running tox locally
• The documentation in docs/ has been updated to cover any changes
• The version in setup.py has been bumped to the next version -- follow semver!
• Add an entry to CHANGELOG.rst has been added

Supersedes and closes #72

[codecov-io]

Codecov Report

Merging #83 into master will decrease coverage by 0.34%.
The diff coverage is 84%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   96.49%   96.14%   -0.35%     
==========================================
  Files          11       11              
  Lines         970      987      +17     
==========================================
+ Hits          936      949      +13     
- Misses         34       38       +4

Impacted Files	Coverage Δ
dynamorm/exceptions.py	100% <100%> (ø)	⬆️
dynamorm/table.py	94.94% <83.33%> (-0.67%)	⬇️

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