Implement new_session functionality

[panagiks]

Closes #281 .

Implements new_session functionality,

Update documentation to include new function.

Update demo code to use new_session for login view.

[codecov]

Codecov Report

Merging #282 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
+ Coverage   96.92%   97.08%   +0.16%     
==========================================
  Files           4        4              
  Lines         195      206      +11     
  Branches       23       25       +2     
==========================================
+ Hits          189      200      +11     
  Misses          4        4              
  Partials        2        2

Impacted Files	Coverage Δ
aiohttp_session/__init__.py	98.05% <100%> (+0.14%)	⬆️

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[panagiks]

I decided to make new_session a regular function (not a coroutine) since there is no reading/writing to the Storage at that point just the creation of an object and placement in the request dictionary.

If there's on objection with that implementation (i.e. you'd prefer to have it as a coroutine for uniformity with get_session) let me know :)

[asvetlov]

I prefer a coroutine.

1. If we'll need a coroutine in a future it's hard to convert a public API
2. Making both get_session() and new_session() coroutines less confuses users.

[panagiks]

Ok, I'll make the change (code and docs) in the evening :)

[panagiks]

Should be good now. Let me know if any further changes are required :)

[asvetlov]

Thanks.
2.5.0 released