Store mailbox names in a map instead of a slice.

Also, delete mailbox names from `:info:` when abandoning a mailbox.
Write a test.

It confirms that the mailbox is removed from `:info:` when it is abandoned.
Use the proper storage with http. Fixes #6
Merge pull request #7 from betawaffle/store-mailbox-names-in-map

Store mailbox names in a map instead of a slice.