Add `replace` script

Find and replace by a given list of files.
Use ag instead of ack

https://github.com/ggreer/the_silver_searcher

* ag is faster than ack
* ag searches all files by default (but still ignores gitignored files). This
  removes the need for ack's `--type-add=` options.
* The command is 33% shorter than ack!
g alias for git

With arguments, g acts like git.
Without arguments, it runs `git status`.

* Source functions/* after loading ZSH completion so `compdef` is available to g
  alias
Explain what's in dotfiles

* Link to fork/upstream process.
* Remove RVM reference.
* Don't create ~/.README.md in `install.sh` script.
Fix contributors link in README
Rename current file in vim buffer

* Retain relative path.
* Save file.
* Do not keep old file open in buffer.

Usage:

    :e old_file_name.rb
    :Rename new_file_name.rb
Change the 'git commit --amend' alias to 'git ca'