Use existsSync in Tunnel.isDownloaded

Fixes #5
Add contribution guidelines
Fix incorrect verbose flag in SauceLabsTunnel. Closes #3
Update to Sauce Connect 4.3. Closes #4
Update Tunnel to use dojo/request for HTTP proxy support

Fixes #1. Closes #9.
Update dojo2 dependency
Updating metadata for 1.1.0
Updating source version to 1.2.0-pre
Update contribution guidelines
Adds support for the '--tunnel-domains' with Sauce Labs