Add a self-update command

[dgil]

Currently we have no way to update captain using captain itself so if we want to have the last version we have to check Github and install the last version manually.

I'd like to propose a new command captain self-update that download the last version (if any) and update the binary in the computer.

In order to have captain self-update we should:

• store the binaries in $HOMEDIR/.captain/binaries/captain-{captain version}
• have a symlink $HOMEDIR/.captain/bin/captain that points to the last binary (this dir should be in the users's $PATH)
• store in github the last version of captain. E.g. https://raw.githubusercontent.com/harbur/captain/master/VERSION

How should it work?

1. User run captain self-update
2. captain check the last version in https://raw.githubusercontent.com/harbur/captain/master/VERSION
3. (if new version available) download the last version and store it in $HOMEDIR/.captain/binaries/captain-{version}
4. Update captain symlink in $HOMEDIR/.captain/bin/

What do you think? Have I missed something?