Vectorize webshot

[FrancoisGuillem]

Hello,

I want to convert a lot of html pages generated with dygraphs in order to generate a gif animation. But for now, each call to webshot takes about three seconds to execute.

After doing a few tests, it looks like most of the time is spent to initialize phantomjs. I think it would save a lot of time to be able to pass a vector of URL and file names to the script webshot.js and to use casper.each to do the job.

[wch]

Closed by #33.