Installation script

[orsinium]

We need a shell script that will:

1. Download the latest release of firefly_cli from github
2. Extract the binary from the archive.
3. Find a writable directory in $PATH. If firefly_cli is already installed, prefer using the same directory.
4. If there is no such directory, create one and add it into PATH through ~/.bashrc or ~/.zshrc
5. Move the binary into the directory
6. chmod +x the binary
7. Create ff alias.

Must work on Linux and OS X. Windows support can be added later.

Prefer sh script. A Python script is also an option but not that portable.