Fix missing variable in threads list

An error was caused in the `threads` function by the the `emails` variable not having been defined as a list before being appended to.  This is just a quick fix - tested and worked for me in Python 2.7

Old behavior:

    >>> print g.inbox().threads()
    NameError: global name 'emails' is not defined

New behavior:

    >>> print g.inbox().threads()
    <gmail.message.Message instance at 0x107752170>, <gmai....