message: Update readme. Add prepare-execute functionality
diff: @@ -197,11 +197,11 @@ you need to start Session first.
  ```rust
  -let use_query_string = String::from("USE my_namespace;");
  +let create_query: Query = QueryBuilder::new("USE my_namespace;").finalize();
  let with_tracing = false;
  let with_warnings = false;
  -match session.query(use_query_string, with_tracing, with_warnings) {
  +match session.query(create_query, with_tracing, with_warnings) {
  Ok(set_keyspace) => {
  // use_keyspace is a result frame of type SetKeyspace
  },
  @@ -217,7 +217,7 @@ that contain namespace and a name of created table.
  ```rust
  use std::default::Default;
  -use cdrs::client::{Query, QueryBuilder};
  +use cdrs::query::{Query, QueryBuilder};
  use cdrs::consistency::Consistency;
  let mut create_query: Query = QueryBuilder::new("CREATE TABLE keyspace.emp (
  @@ -246,7 +246,7 @@ use std::default::Default;
  use cdrs::client::Query;
  use cdrs::consistency::Consistency;
  -let mut select_query: Query = QueryBuilder::new(use_query.clone()).finalize();
  +let select_query: Query = QueryBuilder::new(use_query.clone()).finalize();
  let with_tracing = false;
  let with_warnings = false;
  @@ -325,6 +325,33 @@ let messages: Vec<CAuthor> = rows
  ```
  +##### Prepare and execute a query:
  +
  +Prepare-execute query is also supported:
  +
  +```rust
  + // NOTE: keyspace "keyspace" should already exist
  + let create_table_cql = "USE keyspace;".to_string();
  + let with_tracing = false;
  + let with_warnings = false;
  +
  + // prepare a query
  + let prepared = session.prepare(create_table_cql, with_tracing, with_warnings)
  + .unwrap()
  + .get_body()
  + .into_prepared()
  + .unwrap();
  +
  + // execute prepared query
  + let execution_params = QueryParamsBuilder::new(Consistency::One).finalize();
  + let query_id = prepared.id;
  + let executed = session.execute(query_id, execution_params, false, false)
  + .unwrap()
  + .get_body()
  + .into_set_keyspace()
  + .unwrap();
  +```
  +
  ### License
  The MIT License (MIT)
  