Handle url.Error when vmpooler inaccessible

This release improves error handling in the event of the vmpooler endpoint being inaccessible.

Clicking the short error copies the full text to the clipboard, for example, the full error from above is:

Get https://vmpooler.domain.com/api/v1/vm: dial tcp: lookup vmpooler.domain.com: no such host

Add token generation subcommand

$ vmpooler-bitbar token
Enter vmpooler API endpoint: https://vmpooler.mycompany.net/api/v1
Enter Username: joe.bloggs
Enter Password:

Token generated: pop448v0ztnwta3c964pifngrmk8fake

See the README for more details.

Initial release of Go port

Merge pull request #13 from johnmccabe/FEAT/master/port-to-golang

Port vmpooler-bitbar to Go