Stats: Update JS and GIF domains

Load stats files from cookie-less stats.wp.com and pixel.wp.com domains to
speed up page load.
Merge pull request #812 from xyu/stats-update-domains

Stats: Update JS and GIF domains
REST API: Fix OAuth process.

The OAuth registration process was failing for sites where site_url was
a subfolder.
REST API: Use set_url_scheme.

Use set_url_scheme instead of doing it manually. props @georgestephanis