Implement tuple_type_impl::from_string
Implement tuple_type_impl::from_string

This is a fix for:
https://github.com/scylladb/scylla/issues/574

It mirrors the behavior of:
org.apache.cassandra.db.marshal.TupleType.java#fromString

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <24a7d6253727d0faebb1df117c2f52410523d42f.1459843091.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla