Dynamic only_process support

Enables lambda support for the only_process option to determine which
styles need to be processed delayed dynamically. See the README file
for an example.
[docs] Adding a section about 'url_with_processing.'
[bugfix] I think this needs to call to the class in order to access the
'options' defaults defined there, otherwise it's just accessing the same
'options' (defined on the instance) as the place a few lines down that
 merges the instance options into the default options.

It worked before, because accessing an undefined key in a hash,
returns nil, and the default is also nil, so there was no difference.

However, I want to be able to define a default processing_image_url
that's not nil, so I noticed the problem.
Allowing passing in delayed_paperclip_defaults hash via railties.
Updating readme to discuss updating the default settings ...
config.delayed_paperclip_defaults
[docs] Highlighting the '.processing?' method.
Loosen dependency on sidekiq, support sidekiq 3
Merge pull request #88 from kevinmtrowbridge/delayed_paperclip_defaults

Delayed paperclip defaults
Merge pull request #87 from kevinmtrowbridge/url_with_processing-documentation

[docs] Adding a section about 'url_with_processing.'
Merge pull request #82 from esbeka/feature-dynamic_only_process

Dynamic only_process support
Merge pull request #89 from mrnugget/support_sidekiq_3

Add support for Sidekiq 3.x
Updating Readme and removing ruby files
Merge pull request #92 from ScotterC/remove-ruby-files

Updating Readme and removing ruby files
bump to 2.8.0
bump gemfiles
Merge remote-tracking branch 'upstream/master'

Conflicts:
	delayed_paperclip.gemspec