Add a celiagg kiva backend for Wx

[jwiggins]

Looking towards #414, we should have a celiagg backend for Wx. This PR supplies one.

[rahulporuri]

I see the following tracebacks from the command line when i run the kiva explorer demo in the wx backend + celiagg backend.

> set ETS_TOOLKIT="wx.celiagg"
> python kiva_explorer.py
Traceback (most recent call last):
  File "C:\Users\rporuri\.edm\envs\enable-test-3.6-wx\lib\site-packages\traitsui\wx\code_editor.py", line 375, in _style_needed
    self.control.StartStyling(position, stc.STC_INDIC2_MASK)
TypeError: StyledTextCtrl.StartStyling(): too many arguments
Traceback (most recent call last):
  File "C:\Users\rporuri\.edm\envs\enable-test-3.6-wx\lib\site-packages\traitsui\wx\code_editor.py", line 375, in _style_needed
    self.control.StartStyling(position, stc.STC_INDIC2_MASK)
TypeError: StyledTextCtrl.StartStyling(): too many arguments
Traceback (most recent call last):
  File "C:\Users\rporuri\.edm\envs\enable-test-3.6-wx\lib\site-packages\traitsui\wx\code_editor.py", line 375, in _style_needed
    self.control.StartStyling(position, stc.STC_INDIC2_MASK)
TypeError: StyledTextCtrl.StartStyling(): too many arguments

[rahulporuri]

LGTM. I tested the changes by setting the ETS_TOOLKIT environment variable to wx.celiagg and running the kiva_explorer. Apart from a few unrelated traitsui errors, this seems to do the job.

[jwiggins]

Thanks for the review!