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. Proper feeds support has
also been added for overlay dictionaries.

subpath-2.xml: Alters test to reflect new branch setting method.
maker.py: Adds overlay type checking
maker.py: Implements reload_config() function
maker.py: Adds autofill support for select mirrors

github.com, git.overlays.gentoo.org, and bitbucket.org
mirrors will have their extra source URLs, homepage, and
feeds filled out for them automatically.
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: Adds Exception raising for errors
utils.py: Adds get_input() and get_ans() functions
maker.py: Cleans up large messages
maker.py: Migrates prints to self.output messages
maker.py: Migrates to get_ans & get_input from utils.py