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>
Depending on sqlx makes users pull SQL related code that is usually not
needed. Tools like dep make it easier by explicitly listing relevant
subpackages but the code needs to be downloaded anyway.
