Add support for :meta to the Balanced gateway

Balanced allows for key-value metadata in the `meta` field. The gateway should expose this functionality.

Wrapped up in this pull request is a fix for two broken remote tests and a test for the remote meta functionality.

This pull request is based on [Dan Milliron's fork][dan-milliron] that wasn't ever pulled into the main repo.

[dan-milliron]: https://github.com/demillir/active_merchant