pickler: remove unused import

Signed-off-by: David Aguilar <davvid@gmail.com>
util: avoid calling repr() in is_type()

repr() can be an expensive operation, and can be tickled into an
infinite loop when external code uses jsonpickle in its repr()
implementation.

Avoid the problem entirely by checking the type against types.ClassType,
which is simpler and more robust.

Closes #23

Reported-by: Todd Kennedy (@toddself) <todd.kennedy@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>