Adds delete function at a class level - enables instances of models to be deleted without reading them first

[simonprickett]

This pull request adds a delete function to HashModel and JsonModel - so you can delete something you know the pk for without first having to read it from Redis.

So you can now do... (where Person is a HashModel but could be a JsonModel):

    Person.delete(pk)

before you had to do this:

    person = Person.get(pk)
    person.delete()

I've added a couple of tests too.

Closes #127

[codecov-commenter]

Codecov Report

Merging #132 (6d009a0) into main (2be0174) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   77.20%   77.31%   +0.10%     
==========================================
  Files          12       12              
  Lines        1154     1155       +1     
==========================================
+ Hits          891      893       +2     
+ Misses        263      262       -1     

Flag	Coverage Δ
unit	77.31% <100.00%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aredis_om/model/model.py	86.36% <100.00%> (+0.13%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2be0174...6d009a0. Read the comment docs.