Add spec for passing :scope option
Copy :views option in Sinatra to :view_path option
Pass globaly configured `views` if :views option is blank

This process is same as `views` extraction in the original method.
Remove Sinatra's scope specific switching

There is no need to fetch the views path from `scope` any more because it
is passed via `:view_path` option now, and fetching it is not recommended
because `scope` passed from Sinatra can be non-Sinatra-application instance
like below:

    jbuilder :hello, :scope => Object.new
Remove unused file for spec
Merge pull request #5 from a2ikm/sinatra_view_path

Read Sinatra's views path via :view_path option