Make numpy dependency optional.

ArrayEditor will not be available if numpy is not installed.
Merge pull request #181 from pankajp/optional-numpy

Make numpy dependency optional.
Set the 'shadow' trait first on a ShadowGroup to avoid DelegationErrors.
Merge pull request #185 from enthought/fix/shadow-group-delegation-errors

Set the 'shadow' trait first on a ShadowGroup to avoid DelegationErrors.