Move screenshot code to get image methods.

Before when open url, hoe screen shot image immediately.
So move that code that screen shot when image is needed.
Add probe and capturer class.
Move capture logic to capturer class.
Pool browser process.

Now doesn't close browser after taking picture.
Pool in browserFactory.
Merge remote-tracking branch 'origin/master' into suburl
Make initial phantomjs browsers.
Merge pull request #10 from majecty/browser_pooling

Make browser pooling