Update location of setup_git_author function

- Create new file for all installation setup functions that are only
  used when creating a fresh install
- Move location of setup_git_author from dotfiles script to new
  /lib/setup file

Issue: #14
Create DOTFILES environment variable

I'm finding it more and more valuable to have a global variable that
references the dotfile directory, so that I can use it across subshells
and/or child processes, so create $DOTFILES and change all references to
the old DOTFILES_DIRECTORY accordingly.

**Breaking Change**: Official divergence from the original repository
with this new update!

Issue: #14
Update location of link & mirrorfiles functions

Forgot to add lib/setup file to the 2c0b637 commit, so adding here, and
updating these functions to the new location.

Issue: #14
Update location of change_shell

Since this function is only used for a new install, move to newly
created setup file.

Issue: #14
Update local variable to global env variable

USER_LOG was only used within the utils, but could be valuable in other
use cases, so delete it from location and move it to dotfiles install
script and set it to be a global variable. Give it a smarter name.

Issue: #14
Update GITHUB_USER to global variable

In preparation for using this in other places, change it from locally
scoped variable to environment variable.

Issue: #14
Create function for setting up as git repository

Move functions into setup file in order to clean up dotfiles install
script.

Issue: #14
Update logging to be more consistence

Several places had logging where it didn't make sense, meaning it should
be within the code block so if its run independently or in a separate
place it gets run.

Issue: #11
Create function to install/upgrade Vim bundles

Create a generic function that can take in various flags, only 1
currently, and upgrade as needed.  In this case, it will run the command
line upgrade of Vundle packages.  Upgrade the dotfiles script to use new
command.

Issue: #15