Ensure that Parameterized remaps watchers on copy

[philippjfr]

When a Parameterized is (deep)copied the watchers on it are copied along with the rest of the internal state. The problem with this is that these watchers will still call methods on the original, not on the copy. Therefore we have to override __setstate__ behavior to detect internal watchers (i.e. methods on the instance and those set up by decorating methods using @param.depends(..., watch=True)) and correctly copy them.

Fixes #407

• Add tests

Ping @sdc50

[coveralls]

Coverage increased (+0.2%) to 76.452% when pulling a604900 on philippjfr/407_watcher_copy into f1ea7a9 on master.