Remove Base64 dep

Base64 provides an implementation of `window.btoa` and `window.atob`, but all modern browsers already support this, so let's just use it directly. Base64 was only adding support for IE9 and lower, and `http-browserify` already won’t work in those browsers since it includes `stream` which includes `Buffer` which requires Uint8Array (not supported in IE9 and lower).

This also saves 3KB on bundle size.