new formula: onetime

New formula: onetime. Assists users in creating one-time pads. Still actively supported upstream, and has gathered a lot of renewed interest thanks to Hak5 covering the tool recently.

(*This is my first formula created entirely from scratch, and I'm pretty thrilled to have actually managed to make it work around Homebrew's syntax & formula requirements. Extremely thoroughly tested, so hopefully will pass the build bot first time*).
```
  def install
    inreplace "Makefile", "$(DESTDIR)/usr/bin/", "$(DESTDIR)/#{bin}"
    mkdir bin
    system "make", "install"
```

```
brew install -v --debug /Users/Dominyk/Downloads/onetime.rb
/usr/local/Library/brew.rb (Formulary::FromPathLoader): loading /Users/Dominyk/Downloads/onetime.rb
==> Downloading http://red-bean.com/onetime/onetime-1.81.tar.gz
Already downloaded: /Library/Caches/Homebrew/onetime-1.81.tar.gz
==> Verifying onetime-1.81.tar.gz checksum
tar xf /Library/Caches/Homebrew/onetime-1.81.tar.gz
==> make install
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:1552:in `stat'
Errno::ENOENT: No such file or directory - /usr/local/Cellar/onetime/onetime
```