[telepathy-sasl-signon] Add provenance information to account update flag.

This commit adds a flag to the account which describes which software
component raised the "CredentialsNeedUpdate" flag.
This allows us to determine more easily whether the flag is an old
or new flag, depending on provenance.
Merge pull request #8 from chriadam/master

[telepathy-sasl-signon] Add provenance information to account update fla...