Make cache storage pluggable

[mshibuya]

Refs #461

This pull request introduces abstraction to cache storage strategy and makes it selectable via configuration, thus making it safe to use CarrierWave in a multi-application-server environment(e.g. multiple dyno configuration in Heroku).

You can enable this feature either globally:

CarrierWave.configure do |config|
  config.cache_storage = :fog # default is :file
  config.storage = :fog
end

or per-uploader basis:

class AvatarUploader < CarrierWave::Uploader::Base
  cache_storage :fog
  storage :fog
end

Any suggestions will be greatly appreciated.

Thanks for publishing and maintaining this awesome gem!!

[mshibuya]

Anyone here?
I believe this to be a must-have feature for high volume web applications.

[bensie]

Thanks for this. I'll review it as soon as I have some time, but it's a lot of changes so it may take awhile.

In the meantime, please remove the change to the gemspec -- that has been addressed separately.

[mshibuya]

Thanks for reply, I've rebased it to the master.

[bensie]

What assumptions are being made here? What if there isn't a tmp directory in the working dir?

[mshibuya]

All of supported web application frameworks(Merb, Rails, Sinatra, Padrino) has /tmp directory at the same level with /public, so we can safely assume that ../tmp of CarrierWave.root(=/public) as default location of tmp directory.
If that's not the case or someone wants to use tmp directory other than the framework's default, he or she can specifically set tmp_path such as CarrierWave.tmp_path = '/path/to/tmp' in app's initializer or somewhere around.

[hlxwell]

Wow, when this issue could be merged to master? we really need it.

[mshibuya]

@bensie Any other questions? :)

[hlxwell]

So CI Passed, then will be merged then?

[mshibuya]

@bensie
If you are busy, could you please consider adding me as a collaborator for this project?
I can help fixing problems arising from this change and resolving other issues.

[bensie]

What is a sample value of storage_engines[storage] here and why must it be eval'd?

[mshibuya]

I've just made it the same manner as storage configuration, and the value of storage_engines[storage] can be either "CarrierWave::Storage::File" or "CarrierWave::Storage::Fog" by default.
Tracing back this line revealed that this change was introduced by d1c4156, which states that the purpose is to prevent unnecessary autoloading of classes.

[bensie]

Awesome, thanks for your work on this!

[mshibuya]

Thank you for merging this in!