Various fixes and optimisations to work in the browser

[jeffski]

This pull request addresses the following issues:

• Attaches the Bitpay API to the browser window so it can be instantiated and used out of the box (not dependent on Browserify or any other build tools.
• Uses the correct https endpoint, http was causing XHR preflight redirect errors in the browser.
• Removed nearly all npm dependencies when building for the browser.
• 99% smaller file size making it usable in the browser (+1Mb to 12k when minified).