Move to a base64-based format in the URL for robustness.

This base64 format (with / and + swapped by _ and - respectively)
allows for a more robust URL that avoids misinterpretation (such as a
double-slash) from clients.

H/T to @meqif for this idea.
Fixed the minimum required cURL version that supports SNI
Changed the looks of the README file a bit
Better English 101: Changed from "a HTTP..." to "an HTTP...".

As per RFC 7230, I confirmed it should be "an HTTP..." instead of "a
HTTP...". It even sounds better aloud!

H/T to @raffaelras.
More nitpicking. Now with extra comma(s).
Added a "Goals" section to the README.
More nitpicking.