Refactor (#7)

* Increment nonce when submitting transaction.

* Refactor.

* use default values from metadata
* use functions from metadata

* Nonce fixup

* Add docs.

* Add contracts module.

* Add set_code.

* Add events to metadata.

* Support pretty printing metadata.

* Add subscriptions.

* Add submit_and_watch to XtBuilder.

* nits from review

src/error.rs

@@ -14,17 +14,26 @@
 // You should have received a copy of the GNU General Public License
 // along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
 
+use crate::metadata::MetadataError;
 use jsonrpc_core_client::RpcError;
 use parity_scale_codec::Error as CodecError;
 use std::io::Error as IoError;
 use substrate_primitives::crypto::SecretStringError;
 
+/// Error enum.
 #[derive(Debug, derive_more::From)]
 pub enum Error {
+    /// Codec error.
     Codec(CodecError),
+    /// Io error.
     Io(IoError),
+    /// Rpc error.
     Rpc(RpcError),
+    /// Secret string error.
     SecretString(SecretStringError),
+    /// Metadata error.
+    Metadata(MetadataError),
+    /// Other error.
     Other(String),
 }