Add aah update command to update aah framework to latest version easily

[jeevatkm]

Goal of aah update command is to provide easy and convenient way to update aah framework to the latest version.

So that aah users can simply fire this command after every release. aah update command will update GOPATH/src/aahframework.org/* not a vendor directory.

Note: For vendorized aah application, you should use your package management tool to update your aah to the latest version.

# long command 
aah update

# short command
aah u

• Implementation
• Documentation

[jeevatkm]

It's done.