Added a method to get the thread id

Using the IMAP extensions you can get the thread id of a message.
With this attribute you can easily merge messages while you are fetching them.

see more info about this extension here: https://developers.google.com/google-apps/gmail/imap_extensions#access_to_the_gmail_thread_id_x-gm-thrid
Fix imap extensions in ruby 1.9.3

These constants are defined in Net::IMAP::ResponseParser and
if you don't put the full path to them here it won't work
on ruby 1.9.3
Merge remote-tracking branch 'origin/patch-1'
Optimize message class

With this change only one request to gmail is needed so the data
can be obtained really fast.
Merge pull request #1 from dcu/master

Fix imap extensions