Display the associated account name per buddy.

This change is motivated by the fact I have several instances of the
same buddy in pidgin under different accounts. For example, my buddy
John Smith might be registered under my personal gmail account and my
work gmail account. This change displays the account via the buddy
search provider so you can quickly choose which instance of the buddy
you wish to chat with.
Allow buddies to be filtered by account name too.

If you have the same buddy under different accounts, you can choose the
account you want to use to IM by filtering on it now with a search
term. For example, if you have two accounts with aliases "personal" and
"work", you can IM your buddy John Smith via your work account by typing
"john work" in the overview.
Allow buddies to be filtered by their IM handle.

If you have a buddy that uses more than one IM handle, you can select
the instance you wish to IM by including the handle in the search
term. For example, if your buddy "John Smith" is connected to your gmail
account twice, once via jsmith@gmail and once via jsmobile@gmail.com,
you can select the first by typing "jsmith" in the overview.
Merge pull request #7 from gcla/display_account

Display account