Q:

How can I stage and commit all files, including newly added files, using a single command? 

How can I stage and commit all files, including newly added files, using a single command? 

A:

Does 
git add -A && git commit -m "Your Message"

count as a "single command"? 
Edit based on @thefinnomenon's answer below:
To have it as a git alias, use:
git config --global alias.coa '! git add -A && git commit -m'

and commit all files, including new files, with a message with:
git coa "A bunch of horrible changes"

Explanation (from git add documentation):

-A, --all, --no-ignore-removal
Update the index not only where the working tree has a file matching  but also where the index already has an
  entry.  This adds, modifies, and removes index entries to match the
  working tree. 
If no <pathspec> is given when -A option is used, all files in the
  entire working tree are updated (old versions of Git used to limit the
  update to the current directory and its subdirectories). 

A:

This command will add and commit all the modified files, but not newly created files. 
git commit -am  "<commit message>"

From man git-commit:
   -a, --all
       Tell the command to automatically stage files that have been modified
       and deleted, but new files you have not told Git about are not
       affected. 

A:

I use this function:
gcaa() { git add --all && git commit -m "$*" }

In my zsh config file, so i can just do:
> gcaa This is the commit message

To automatically stage and commit all files.