Add a way to list all gmail labels, and rename labels.

[dcparker]

Add a way to list all gmail labels, and rename labels.

[justinperkins]

I was looking into this too. A typical use-case would be to check if a message has a label, and create/apply it to the message if not. The Net::IMAP#list method gets all the labels, but still unsure how to reconcile which messages have a given label.

[jasonwbarnett]

I added a way to rename and destroy labels in my fork just in case you want it... https://github.com/jasonwbarnett/ruby-gmail

I added these specifically in https://github.com/jasonwbarnett/ruby-gmail/commit/a3bb025b5e7d68e61a384a76a8441b30ba7447ab

[myobie]

I merge a PR in that should add this. I'll cut a gem version tonight.