Soprano Saxophone - Geckodriver install

This is a simple package that downloads Geckodriver and
provides a node api for accessing the path to the binary. There are other packages like this, but I wanted to make sure
I had an updated package to include in Browsertime.

Why the funny name you ask? Well, there are countless Selenium modules in npm.
Keywords and the description for the module allows you to find it anyway, so might as well choose a unique name.

How to use?
``node
var driver = require('soprano-saxophone').geckodriver;

var p = driver.binPath();
// launch geckodriver from path
``

You can ovveride the download location by setting process.env.GECKODRIVER_BASE_URL.