correctly cancel pending connection

Avoid a segfault when disabling the account by correctly cancelling an
in-progress connection
Use portable string format to avoid compiler warnings
Add libpurple feedback to the connection process
Add an option to hide notifications

When using multiple characters, it can be useful to only leave
notifications enabled on the main one, and avoid receiving duplicates.
Merge pull request #1 from vobe/master

Couple of fixes and improvements