Repokey gnome keyring

For crypted libraries is the password stored in the plaintext form in the sqlite3 database. 
I have added support for gnome-keyring to store the repository password instead of the the sqlite3 file.

The support must be enabled in configure time by adding the --enable-gnome-keyring. Also the required dependencies must be added when building the linux package.

At this moment it's easy to implement to store in the GK also the values form the RepoKey database or any other value.

I do not have WIN or MAC development tools to add the same support for these clients.
If someone will try to add also support for these platform it would be good to do some abstraction. Right now there is only simple #ifdef macro.