Add Cache factory

[argaen]

Adds aiocache.Cache as a factory for creating different caches. Previously you would have to import the specific backend as

from aiocache import RedisCache

cache = RedisCache

now

from aiocache import Cache

cache = Cache(Cache.REDIS)

it just adds a level of indirection but will help on adding/centralizing features like creating a cache from a resource url, creating from a string for the config, etc. Also recommending this approach to create caches from now on

[codecov]

Codecov Report

Merging #430 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
+ Coverage   99.77%   99.77%   +<.01%     
==========================================
  Files           9       10       +1     
  Lines         873      894      +21     
  Branches       96       96              
==========================================
+ Hits          871      892      +21     
  Misses          2        2

Impacted Files	Coverage Δ
aiocache/lock.py	100% <ø> (ø)	⬆️
aiocache/serializers.py	96.49% <ø> (ø)	⬆️
aiocache/exceptions.py	100% <100%> (ø)
aiocache/factory.py	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #430 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
+ Coverage   99.77%   99.77%   +<.01%     
==========================================
  Files           9        9              
  Lines         873      882       +9     
  Branches       96       96              
==========================================
+ Hits          871      880       +9     
  Misses          2        2

Impacted Files	Coverage Δ
aiocache/lock.py	100% <ø> (ø)	⬆️
aiocache/serializers.py	96.49% <ø> (ø)	⬆️
aiocache/decorators.py	100% <100%> (ø)	⬆️
aiocache/factory.py	100% <100%> (ø)	⬆️

---

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 699db4e...61ad59c. Read the comment docs.

[argaen]

@tomchristie the idea after this is to add a Cache.from_url that would instantiate a cache from a resource url. What do you think?

[tomchristie]

Yup good stuff! I guess ultimately I'd prefer Cache("redis://localhost/0") over Cache.from_url("redis://localhost/0") but either way this change helps make that possible.

👍