Disable warnings as errors

The commit fixes issue JayBazuzi/Pash2/issues/23 by adding
an attribute to stop xbuild/mono treating warnings as errors.
Merge branch 'fix-warn-error' of https://github.com/b333z/Pash2 into travis2-wx
warn-as-error only on Windows
Merge branch 'travis2-wx' into travis2

# By Andrew Barnes (1) and Jay Bazuzi (1)
* travis2-wx:
  warn-as-error only on Windows
  Disable warnings as errors