Extra configurable options

Hey Mike, thanks for the great module.

I've added some extra configurable options to the library, especially for PDF output. I haven't fully tested nor documented all the changes yet because I wanted your input first. I made some changes in the way arguments are parsed for greater flexibility. The options object literal now looks like this:

    var options = {
        out: './tmp',
        format: 'pdf',
        viewportWidth: 915,
        viewportHeight: 768,
        paperMargin: '2mm',
        paperOrientation: 'portait',
        paperFormat: 'A4',
        phantomBin: __dirname + '/node_modules/phantomjs/bin/phantomjs'
    };