style(str): less string formatting

Substitution is slower then string concatenation in pypy.
feat(set): use OStringStream to build request

StringBuilder in pypy and cStringIO in py27.
feat(type): add pickle support

Pickle new protocol is enforced.

The type code of pickled data is 0, which is TypeCode.Empty in
C#, but EnyimMemcached never uses this type code as flag.