Refactor overriding of IMGKit::Configuration#wkhtmltoimage

Before this change, the initialize method would check the system for
paths to the wkhtmltoimage binary even if it will be overridden later.

With this change, we only check the system if @wkhtmltoimage hasn't been
explicitly set via the attr_writer
Merge pull request #57 from lmars/refactor-configuration

Refactor overriding of IMGKit::Configuration#wkhtmltoimage