Store plClient password using platform-specific credential storage.

Add an abstracted password-storage system, and utilize it in plClient to replace the drive-based obfuscation.
Sorry, there was a bit of discussion in IRC about this, and after looking at the OSX Keychain and Gnome Seahorse it seems like using the shard title is probably the best bet.
Using a UUID results in the credential description being listed as that UUID, which doesn't provide any indication as to what the credential is for.