Merge pull request #3 from hailocab/upstream-merge

Upstream merge
Add Session wrapper

With this patch we can now use gocqlx like:

```
session.Query(`SELECT * FROM struct_table`, nil).Get(&v)
```

instead of (old format):

```
gocqlx.Query(session.Query(`SELECT * FROM struct_table`), nil).Get(&v)
```

Signed-off-by: Michał Matczuk <michal@scylladb.com>

session.go

@@ -0,0 +1,60 @@
+// Copyright (C) 2017 ScyllaDB
+// Use of this source code is governed by a ALv2-style
+// license that can be found in the LICENSE file.
+
+package gocqlx
+
+import (
+	"context"
+
+	"github.com/gocql/gocql"
+	"github.com/scylladb/go-reflectx"
+)
+
+// Session wraps gocql.Session and provides a modified Query function that
+// returns Queryx instance.
+// The original Session instance can be accessed as Session.
+// The default mapper uses `db` tag and automatically converts struct field
+// names to snake case. If needed package reflectx provides constructors
+// for other types of mappers.
+type Session struct {
+	*gocql.Session
+	Mapper *reflectx.Mapper
+}
+
+// WrapSession should be called on CreateSession() gocql function to convert
+// the created session to gocqlx.Session.
+func WrapSession(session *gocql.Session, err error) (Session, error) {
+	return Session{
+		Session: session,
+		Mapper:  DefaultMapper,
+	}, err
+}
+
+// ContextQuery is a helper function that allows to pass context when creating
+// a query, see the "Query" function .
+func (s Session) ContextQuery(ctx context.Context, stmt string, names []string) *Queryx {
+	return &Queryx{
+		Query:  s.Session.Query(stmt).WithContext(ctx),
+		Names:  names,
+		Mapper: s.Mapper,
+	}
+}
+
+// Query creates a new Queryx using the session mapper.
+// The stmt and names parameters are typically result of a query builder
+// (package qb) ToCql() function or come from table model (package table).
+// The names parameter is a list of query parameters' names and it's used for
+// binding.
+func (s Session) Query(stmt string, names []string) *Queryx {
+	return &Queryx{
+		Query:  s.Session.Query(stmt),
+		Names:  names,
+		Mapper: s.Mapper,
+	}
+}
+
+// ExecStmt creates query and executes the given statement.
+func (s Session) ExecStmt(stmt string) error {
+	return s.Query(stmt, nil).ExecRelease()
+}