Add Chronic support for modifiedSince in conversations

Hated always having to put dates into this UTC format (without the "UTC"). Saw your "TODO" and thought be able to help.

Seemed like Chronic would make queries like this easier:

    conversations = HelpScout::Base.conversations(mailbox.id, "all", "last week")

And would also handle dates mistakenly entered without the required **H** and **Z**.

I'm assuming the HelpScout guys are planning to add more `modifiedSince` params into their queries in the future, so this will be useful for more than just one method then :smile: