Unicode trait handling for those vtk methods returning unicode data

[stefanoborini]

The generator code did not handle unicode, but some vtk routines do return them. Although never able to reproduce it, issue #354 shows a situation where it does matter.

I am unsure about potential side effects of this change on client code, but I am surprised it worked until now, and I expect the method was simply not generated, or it was but it returned an empty string.

[rkern]

if default == u'\x00':
    default = u''
t_def = 'traits.Unicode({0!r}, enter_set=True, auto_set=False)'.format(default)

[rkern]

Don't .encode() or clean_special_chars(). {!r} takes care of everything.

[stefanoborini]

@rkern done. I also took care of the part handling String and removed the cleaning function.

[kitchoi]

Verified that it works with the following tox.ini

[tox]
envlist = py27

[testenv]
# To make sure we actually test the tarball and now the working directory
changedir = .tox

# We use symbolic links to provide vtk which is not on pypi
whitelist_externals=/bin/ln

# Install development requirements
deps = numpy

# e.g. export VTK_PATH=${HOME}/.cache/VTK-6.3.0-Linux-64bit/lib/python2.7/site-packages/vtk
# e.g. export LD_LIBRARY_PATH=${HOME}/.cache/VTK-6.3.0-Linux-64bit/lib
passenv = VTK_PATH LD_LIBRARY_PATH

basepython = python2.7

commands =
     ln -sf {env:VTK_PATH} {envsitepackagesdir}/

[stefanoborini]

Failure seems due to a network problem. Restarting

[rkern]

default = u''