If wkhtmltoimage doesn't complete in 2.0 seconds, the process is killed and control returns to the calling code. This was needed because wbhtmltoimage hangs often, even after successful image captures.
Since STDIN and STDOUT are unreliable when a process is killed, now communicates with  wkhtmltoimage via tempfiles.
Merge branch 'master' of https://github.com/thecurator/IMGKit

Conflicts:
	lib/imgkit/configuration.rb
	lib/imgkit/imgkit.rb
	spec/imgkit_spec.rb