optimization: eliminate a double call to the method url() and store result in a local variable

The url() method when defined in an engine class typically contains logic and dynamic string allocation.
and it annoyed me.

may be url_result is not the best name but that is the best I came up with.
Merge pull request #234 from lidlanca/patch-3

optimization: eliminate a double call to the method url() and store result in a local variable
Add Engine to support github issues
github_issue_onebox.rb  matcher rule to be more specific.
Merge pull request #235 from lidlanca/github_issue_onebox

Add Engine to support github issues
The only one I dont need
add onebox-avatar class to enforce proper sizing 

https://avatars.githubusercontent.com may fail to provide a 96x96 avatars even when specifying  s=96 in the URL
on auto generated avatars. uploaded avatars are fine.
Merge pull request #236 from lidlanca/github_issue_onebox

The only one I dont need
add options for each engine. extracted from global setting Onebox.options based on class name

To set options for the engine github_blob_onebox.rb 
Onebox.options = {
  GithubBlobOnebox => {:SHOW_LINE_NUMBER => true}
}

then from within the engine instance engine options can be accessed 
self.options[:SHOW_LINE_NUMBER]
or get all options using
Onebox.options
Merge pull request #237 from lidlanca/patch-4

add options for each engine. extracted from Onebox.options based on class name
Template to support new pull statuses
implement api authentications and get build status

oauth is the recommend authentication method. when generating token you can control privileges, and you do not expose your password

authentication method and crediential is provided in the engine options. see comments of the method api_json_request()
Update engine.rb
Onebox Github Blob - Add support for line numbers. indentation reducer
match super  initialize
Merge pull request #239 from lidlanca/onebox-github-blob2

Onebox Github Blob -Fix:Line Range Index.  Add:support for line numbers. Add:indentation reducer optimization
Merge pull request #238 from lidlanca/github_api_auth

Github api auth. Github Pull request engine improvements to get pull and build status
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