overlay.py: Removes old branch tag components

In favor of setting the branch variable in each source tag, the old
branch tag components have been removed.

subpath-2.xml: Alters test to reflect new branch setting method.
Adds multiple description field support

X-Gentoo-Bug: 465730
X-Gentoo-Bug-URL: https://bugs.gentoo.org/465730
maker.py: Adds stub function to grab descriptions

This function has been added in hopes of later adding multiple
description field support to the interactive overlay creation
session.
maker.py: Notifies user of location of successful write
maker.py: Adds "https://" source splitting support
maker.py: Adds Exception raising for errors
maker.py: Modifies auto-filled source URL unpacking
maker.py: Organizes auto-complete function
maker.py: Adds additional info support bitbucket.org mirrors