allow Get() calls to avoid copies into std::string

Add a new abstract base class leveldb::Value that applications can easily
derive from to supply their own memory management for values retrieved via
Get(). Add an internal class derived from Value that provides std::string
management to preserve backward compatibility. Overload DBImpl::Get() to
accept a Value*, and to preserve backward compatibility also keep the
original version taking a std::string*.
Merge pull request #72 from basho/sbv-db-value-holder

allow Get() calls to avoid copies into std::string