Adds layman-overlay-maker tool

This tool has been made to assist users in creating their own
layman overlay definitions.

constants.py: Adds overlay component constants.
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.
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: 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