Fix #80 - global refactor

[moul]

What I have done:

• moved the main.go file in the ./cmd/scw/ folder
• moved all the packes (api, scwversion, utils, commands) in the ./pkg/ folder
• split the commands code between 2 packages:
  • cli: command-line parsing, usage, help, and exit codes
  • commands: actions without notion of command-line interface, everything is run in a context (own stdin,stdout,stderr), no exits

We will now be able to chain and write unit tests for the commands

---

Done:

• global project structure
• command help
• command attach
• command commit
• command cp
• command create
• command events
• command exec
• command history
• command images
• command info
• command inspect
• command kill
• command login
• command logout
• command logs
• command port
• command ps
• command rename
• command restart
• command rm
• command rmi
• command run
• command search
• command start
• command stop
• command tag
• command top
• command version
• command wait

[QuentinPerez]

LGTM 👍 Good work 😊