Moved configuration around for validator client.

- Custom datadir/server argument logic moved into configuration, out of main.
 - Updated the validator config directory structure, as per issue #253 suggestions
 - Removed the 'generate 3 random keys' function
 - Updated the README to reflect new structure
 - Just exit if there are no keys, don't generate any (this is for accounts_manager, in a separate commit).
 - Created a lib.rs file, so that the validator client configuration can be included by external crates.

validator_client/src/lib.rs

@@ -0,0 +1,3 @@
+pub mod config;
+
+pub use crate::config::ValidatorClientConfig;