Add #localize to Gmail::Labels

Remembering localizations for each mailbox name is a pain; Gmail has very
little consistency on how they name their folders as well. Some are prefixes
by [Gmail], others by [Google Mail]; Some trash folders are called "Trash",
others "Bin", yet others are localized into other languages.

This commit leverages Gmail's support of the XLIST command to use
standardized, non-localized names for specific folders to find the correct
labels. The supported list is:
  * :Inbox
  * :Allmail
  * :Drafts
  * :Sent
  * :Trash
  * :Important
  * :Spam

However, even these are not all present on every account. If a name is passed
that *should* be recognized, but isn't, the localizer will return nil. If a
non-recognized name is passed, it is returned as-is.
Update README with #localize
Update #localize to use LIST special-use format

The XLIST extension has been depreciated by Google, with the replacement being
the LIST special-use extension. Syntax is almost exactly the same; the only
two changes are:

  * Flagged now denotes Starred messages
  * Spam is now called Junk

Otherwise, operation should be exactly the same.
Merge remote-tracking branch 'upstream/localizing-labels'