Add model manager for deleting old job executions.

[jcass77]

Adds a new model manager for DjangoJobExecution to allow deleting old job executions, which can tally up quite quickly if multiple jobs have been scheduled with a high level of frequency.

• Not sure why GitHub generated the large diff on the README: I only added one bullet point to describe the new feature.
• I had to move the tests around a bit so that we could re-use some fixtures and mock objects centrally from conftest.py.

[codecov-io]

Codecov Report

Merging #58 into master will decrease coverage by 0.43%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   72.69%   72.25%   -0.44%     
==========================================
  Files           9        9              
  Lines         304      310       +6     
==========================================
+ Hits          221      224       +3     
- Misses         83       86       +3

Impacted Files	Coverage Δ
django_apscheduler/models.py	81.81% <100%> (-8.19%)	⬇️
django_apscheduler/jobstores.py	73.79% <0%> (+2.06%)	⬆️

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[jarekwg]

Thanks for the PR. Would you be interested in helping maintain this package?

[jarekwg]

Published in 0.3.0.